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Side-Lights on Astronomy and Kindred Fields of Popular Science

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Title: Side-Lights on Astronomy and Kindred Fields of Popular Science

Author: Simon Newcomb


        
Release date: May 1, 2003 [eBook #4065]
                Most recently updated: December 27, 2020

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK SIDE-LIGHTS ON ASTRONOMY AND KINDRED FIELDS OF POPULAR SCIENCE ***




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SIDE-LIGHTS ON ASTRONOMY

AND KINDRED FIELDS OF POPULAR SCIENCE


ESSAYS AND ADDRESSES


BY

SIMON NEWCOMB




CONTENTS


PREFACE

     I. THE UNSOLVED PROBLEMS OF ASTRONOMY
    II. THE NEW PROBLEMS OF THE UNIVERSE
   III. THE STRUCTURE OF THE UNIVERSE
    IV. THE EXTENT OF THE UNIVERSE
     V. MAKING AND USING A TELESCOPE
    VI. WHAT THE ASTRONOMERS ARE DOING
   VII. LIFE IN THE UNIVERSE
  VIII. HOW THE PLANETS ARE WEIGHED
    IX. THE MARINER'S COMPASS
     X. THE FAIRYLAND OF GEOMETRY
    XI. THE ORGANIZATION OF SCIENTIFIC RESEARCH
   XII. CAN WE MAKE IT RAIN?
  XIII. THE ASTRONOMICAL EPHEMERIS AND NAUTICAL ALMANAC
   XIV. THE WORLD'S DEBT TO ASTRONOMY
    XV. AN ASTRONOMICAL FRIENDSHIP
   XVI. THE EVOLUTION OF THE SCIENTIFIC INVESTIGATOR
  XVII. THE EVOLUTION OF ASTRONOMICAL KNOWLEDGE
 XVIII. ASPECTS OF AMERICAN ASTRONOMY
   XIX. THE UNIVERSE AS AN ORGANISM
    XX. THE RELATION OF SCIENTIFIC METHOD TO SOCIAL PROGRESS
   XXI. THE OUTLOOK FOR THE FLYING-MACHINE




ILLUSTRATIONS

SIMON NEWCOMB

PHOTOGRAPH OF THE CORONA OF THE SUN, TAKEN IN TRIPOLI DURING TOTAL
ECLIPSE OF AUGUST 30, 1905.

A TYPICAL STAR CLUSTER-CENTAURI

THE GLASS DISK

THE OPTICIAN'S TOOL

THE OPTICIAN'S TOOL

GRINDING A LARGE LENS

IMAGE OF CANDLE-FLAME IN OBJECT-GLASS

TESTING ADJUSTMENT OF OBJECT-GLASS

A VERY PRIMITIVE MOUNTING FOR A TELESCOPE

THE HUYGHENIAN EYE-PIECE

SECTION OF THE PRIMITIVE MOUNTING

SPECTRAL IMAGES OF STARS, THE UPPER LINE SHOWING HOW THEY APPEAR WITH
THE EYE-PIECE PUSHED IN, THE LOWER WITH THE EYE-PIECE DRAWN OUT

THE GREAT REFRACTOR OF THE NATIONAL OBSERVATORY AT WASHINGTON

THE "BROKEN-BACKED COMET-SEEKER"

NEBULA IN ORION

DIP OF THE MAGNETIC NEEDLE IN VARIOUS LATITUDES

STAR SPECTRA

PROFESSOR LANGLEY'S AIR-SHIP






PREFACE

In preparing and issuing this collection of essays and addresses, the
author has yielded to what he could not but regard as the too
flattering judgment of the publishers. Having done this, it became
incumbent to do what he could to justify their good opinion by revising
the material and bringing it up to date. Interest rather than unity of
thought has determined the selection.

A prominent theme in the collection is that of the structure, extent,
and duration of the universe. Here some repetition of ideas was found
unavoidable, in a case where what is substantially a single theme has
been treated in the various forms which it assumed in the light of
constantly growing knowledge. If the critical reader finds this a
defect, the author can plead in extenuation only the difficulty of
avoiding it under the circumstances. Although mainly astronomical, a
number of discussions relating to general scientific subjects have been
included.

Acknowledgment is due to the proprietors of the various periodicals
from the pages of which most of the essays have been taken. Besides
Harper's Magazine and the North American Review, these include
McClure's Magazine, from which were taken the articles "The Unsolved
Problems of Astronomy" and "How the Planets are Weighed." "The
Structure of the Universe" appeared in the International Monthly, now
the International Quarterly; "The Outlook for the Flying-Machine" is
mainly from The New York Independent, but in part from McClure's
Magazine; "The World's Debt to Astronomy" is from The Chautauquan; and
"An Astronomical Friendship" from the Atlantic Monthly.

SIMON NEWCOMB. WASHINGTON, JUNE, 1906.




I

THE UNSOLVED PROBLEMS OF ASTRONOMY


The reader already knows what the solar system is: an immense central
body, the sun, with a number of planets revolving round it at various
distances. On one of these planets we dwell. Vast, indeed, are the
distances of the planets when measured by our terrestrial standards. A
cannon-ball fired from the earth to celebrate the signing of the
Declaration of Independence, and continuing its course ever since with
a velocity of eighteen hundred feet per second, would not yet be
half-way to the orbit of Neptune, the outer planet. And yet the
thousands of stars which stud the heavens are at distances so much
greater than that of Neptune that our solar system is like a little
colony, separated from the rest of the universe by an ocean of void
space almost immeasurable in extent. The orbit of the earth round the
sun is of such size that a railway train running sixty miles an hour,
with never a stop, would take about three hundred and fifty years to
cross it. Represent this orbit by a lady's finger-ring. Then the
nearest fixed star will be about a mile and a half away; the next more
than two miles; a few more from three to twenty miles; the great body
at scores or hundreds of miles. Imagine the stars thus scattered from
the Atlantic to the Mississippi, and keep this little finger-ring in
mind as the orbit of the earth, and one may have some idea of the
extent of the universe.

One of the most beautiful stars in the heavens, and one that can be
seen most of the year, is a Lyrae, or Alpha of the Lyre, known also as
Vega. In a spring evening it may be seen in the northeast, in the later
summer near the zenith, in the autumn in the northwest. On the scale we
have laid down with the earth's orbit as a finger-ring, its distance
would be some eight or ten miles. The small stars around it in the same
constellation are probably ten, twenty, or fifty times as far.

Now, the greatest fact which modern science has brought to light is
that our whole solar system, including the sun, with all its planets,
is on a journey towards the constellation Lyra. During our whole lives,
in all probability during the whole of human history, we have been
flying unceasingly towards this beautiful constellation with a speed to
which no motion on earth can compare. The speed has recently been
determined with a fair degree of certainty, though not with entire
exactness; it is about ten miles a second, and therefore not far from
three hundred millions of miles a year. But whatever it may be, it is
unceasing and unchanging; for us mortals eternal. We are nearer the
constellation by five or six hundred miles every minute we live; we are
nearer to it now than we were ten years ago by thousands of millions of
miles, and every future generation of our race will be nearer than its
predecessor by thousands of millions of miles.

When, where, and how, if ever, did this journey begin--when, where, and
how, if ever, will it end? This is the greatest of the unsolved
problems of astronomy. An astronomer who should watch the heavens for
ten thousand years might gather some faint suggestion of an answer, or
he might not. All we can do is to seek for some hints by study and
comparison with other stars.

The stars are suns. To put it in another way, the sun is one of the
stars, and rather a small one at that. If the sun is moving in the way
I have described, may not the stars also be in motion, each on a
journey of its own through the wilderness of space? To this question
astronomy gives an affirmative answer. Most of the stars nearest to us
are found to be in motion, some faster than the sun, some more slowly,
and the same is doubtless true of all; only the century of accurate
observations at our disposal does not show the motion of the distant
ones. A given motion seems slower the more distant the moving body; we
have to watch a steamship on the horizon some little time to see that
she moves at all. Thus it is that the unsolved problem of the motion of
our sun is only one branch of a yet more stupendous one: What mean the
motions of the stars--how did they begin, and how, if ever, will they
end? So far as we can yet see, each star is going straight ahead on its
own journey, without regard to its neighbors, if other stars can be so
called. Is each describing some vast orbit which, though looking like a
straight line during the short period of our observation, will really
be seen to curve after ten thousand or a hundred thousand years, or
will it go straight on forever? If the laws of motion are true for all
space and all time, as we are forced to believe, then each moving star
will go on in an unbending line forever unless hindered by the
attraction of other stars. If they go on thus, they must, after
countless years, scatter in all directions, so that the inhabitants of
each shall see only a black, starless sky.

Mathematical science can throw only a few glimmers of light on the
questions thus suggested. From what little we know of the masses,
distances, and numbers of the stars we see a possibility that the more
slow-moving ones may, in long ages, be stopped in their onward courses
or brought into orbits of some sort by the attraction of their millions
of fellows. But it is hard to admit even this possibility in the case
of the swift-moving ones. Attraction, varying as the inverse square of
the distance, diminishes so rapidly as the distance increases that, at
the distances which separate the stars, it is small indeed. We could
not, with the most delicate balance that science has yet invented, even
show the attraction of the greatest known star. So far as we know, the
two swiftest-moving stars are, first, Arcturus, and, second, one known
in astronomy as 1830 Groombridge, the latter so called because it was
first observed by the astronomer Groombridge, and is numbered 1830 in
his catalogue of stars. If our determinations of the distances of these
bodies are to be relied on, the velocity of their motion cannot be much
less than two hundred miles a second. They would make the circuit of
the earth every two or three minutes. A body massive enough to control
this motion would throw a large part of the universe into disorder.
Thus the problem where these stars came from and where they are going
is for us insoluble, and is all the more so from the fact that the
swiftly moving stars are moving in different directions and seem to
have no connection with each other or with any known star.

It must not be supposed that these enormous velocities seem so to us.
Not one of them, even the greatest, would be visible to the naked eye
until after years of watching. On our finger-ring scale, 1830
Groombridge would be some ten miles and Arcturus thirty or forty miles
away. Either of them would be moving only two or three feet in a year.
To the oldest Assyrian priests Lyra looked much as it does to us
to-day. Among the bright and well-known stars Arcturus has the most
rapid apparent motion, yet Job himself would not to-day see that its
position had changed, unless he had noted it with more exactness than
any astronomer of his time.

Another unsolved problem among the greatest which present themselves to
the astronomer is that of the size of the universe of stars. We know
that several thousand of these bodies are visible to the naked eye;
moderate telescopes show us millions; our giant telescopes of the
present time, when used as cameras to photograph the heavens, show a
number past count, perhaps one hundred millions. Are all these stars
only those few which happen to be near us in a universe extending out
without end, or do they form a collection of stars outside of which is
empty infinite space? In other words, has the universe a boundary?
Taken in its widest scope this question must always remain unanswered
by us mortals because, even if we should discover a boundary within
which all the stars and clusters we ever can know are contained, and
outside of which is empty space, still we could never prove that this
space is empty out to an infinite distance. Far outside of what we call
the universe might still exist other universes which we can never see.

It is a great encouragement to the astronomer that, although he cannot
yet set any exact boundary to this universe of ours, he is gathering
faint indications that it has a boundary, which his successors not many
generations hence may locate so that the astronomer shall include
creation itself within his mental grasp. It can be shown mathematically
that an infinitely extended system of stars would fill the heavens with
a blaze of light like that of the noonday sun. As no such effect is
produced, it may be concluded that the universe has a boundary. But
this does not enable us to locate the boundary, nor to say how many
stars may lie outside the farthest stretches of telescopic vision. Yet
by patient research we are slowly throwing light on these points and
reaching inferences which, not many years ago, would have seemed
forever beyond our powers.

Every one now knows that the Milky Way, that girdle of light which
spans the evening sky, is formed of clouds of stars too minute to be
seen by the unaided vision. It seems to form the base on which the
universe is built and to bind all the stars into a system. It comprises
by far the larger number of stars that the telescope has shown to
exist. Those we see with the naked eye are almost equally scattered
over the sky. But the number which the telescope shows us become more
and more condensed in the Milky Way as telescope power is increased.
The number of new stars brought out with our greatest power is vastly
greater in the Milky Way than in the rest of the sky, so that the
former contains a great majority of the stars. What is yet more
curious, spectroscopic research has shown that a particular kind of
stars, those formed of heated gas, are yet more condensed in the
central circle of this band; if they were visible to the naked eye, we
should see them encircling the heavens as a narrow girdle forming
perhaps the base of our whole system of stars. This arrangement of the
gaseous or vaporous stars is one of the most singular facts that modern
research has brought to light. It seems to show that these particular
stars form a system of their own; but how such a thing can be we are
still unable to see.

The question of the form and extent of the Milky Way thus becomes the
central one of stellar astronomy. Sir William Herschel began by trying
to sound its depths; at one time he thought he had succeeded; but
before he died he saw that they were unfathomable with his most
powerful telescopes. Even today he would be a bold astronomer who would
profess to say with certainty whether the smallest stars we can
photograph are at the boundary of the system. Before we decide this
point we must have some idea of the form and distance of the cloudlike
masses of stars which form our great celestial girdle. A most curious
fact is that our solar system seems to be in the centre of this
galactic universe, because the Milky Way divides the heavens into two
equal parts, and seems equally broad at all points. Were we looking at
such a girdle as this from one side or the other, this appearance would
not be presented. But let us not be too bold. Perhaps we are the
victims of some fallacy, as Ptolemy was when he proved, by what looked
like sound reasoning, based on undeniable facts, that this earth of
ours stood at rest in the centre of the heavens!

A related problem, and one which may be of supreme importance to the
future of our race, is, What is the source of the heat radiated by the
sun and stars? We know that life on the earth is dependent on the heat
which the sun sends it. If we were deprived of this heat we should in a
few days be enveloped in a frost which would destroy nearly all
vegetation, and in a few months neither man nor animal would be alive,
unless crouching over fires soon to expire for want of fuel. We also
know that, at a time which is geologically recent, the whole of New
England was covered with a sheet of ice, hundreds or even thousands of
feet thick, above which no mountain but Washington raised its head. It
is quite possible that a small diminution in the supply of heat sent us
by the sun would gradually reproduce the great glacier, and once more
make the Eastern States like the pole. But the fact is that
observations of temperature in various countries for the last two or
three hundred years do not show any change in climate which can be
attributed to a variation in the amount of heat received from the sun.

The acceptance of this theory of the heat of those heavenly bodies
which shine by their own light--sun, stars, and nebulae--still leaves
open a problem that looks insoluble with our present knowledge. What
becomes of the great flood of heat and light which the sun and stars
radiate into empty space with a velocity of one hundred and eighty
thousand miles a second? Only a very small fraction of it can be
received by the planets or by other stars, because these are mere
points compared with their distance from us. Taking the teaching of our
science just as it stands, we should say that all this heat continues
to move on through infinite space forever. In a few thousand years it
reaches the probable confines of our great universe. But we know of no
reason why it should stop here. During the hundreds of millions of
years since all our stars began to shine, has the first ray of light
and heat kept on through space at the rate of one hundred and eighty
thousand miles a second, and will it continue to go on for ages to
come? If so, think of its distance now, and think of its still going
on, to be forever wasted! Rather say that the problem, What becomes of
it? is as yet unsolved.

Thus far I have described the greatest of problems; those which we may
suppose to concern the inhabitants of millions of worlds revolving
round the stars as much as they concern us. Let us now come down from
the starry heights to this little colony where we live, the solar
system. Here we have the great advantage of being better able to see
what is going on, owing to the comparative nearness of the planets.
When we learn that these bodies are like our earth in form, size, and
motions, the first question we ask is, Could we fly from planet to
planet and light on the surface of each, what sort of scenery would
meet our eyes? Mountain, forest, and field, a dreary waste, or a
seething caldron larger than our earth? If solid land there is, would
we find on it the homes of intelligent beings, the lairs of wild
beasts, or no living thing at all? Could we breathe the air, would we
choke for breath or be poisoned by the fumes of some noxious gas?

To most of these questions science cannot as yet give a positive
answer, except in the case of the moon. Our satellite is so near us
that we can see it has no atmosphere and no water, and therefore cannot
be the abode of life like ours. The contrast of its eternal deadness
with the active life around us is great indeed. Here we have weather of
so many kinds that we never tire of talking about it. But on the moon
there is no weather at all. On our globe so many things are constantly
happening that our thousands of daily journals cannot begin to record
them. But on the dreary, rocky wastes of the moon nothing ever happens.
So far as we can determine, every stone that lies loose on its surface
has lain there through untold ages, unchanged and unmoved.

We cannot speak so confidently of the planets. The most powerful
telescopes yet made, the most powerful we can ever hope to make, would
scarcely shows us mountains, or lakes, rivers, or fields at a distance
of fifty millions of miles. Much less would they show us any works of
man. Pointed at the two nearest planets, Venus and Mars, they whet our
curiosity more than they gratify it. Especially is this the case with
Venus. Ever since the telescope was invented observers have tried to
find the time of rotation of this planet on its axis. Some have reached
one conclusion, some another, while the wisest have only doubted. The
great Herschel claimed that the planet was so enveloped in vapor or
clouds that no permanent features could be seen on its surface. The
best equipped recent observers think they see faint, shadowy patches,
which remain the same from day to day, and which show that the planet
always presents the same face to the sun, as the moon does to the
earth. Others do not accept this conclusion as proved, believing that
these patches may be nothing more than variations of light, shade, and
color caused by the reflection of the sun's light at various angles
from different parts of the planet.

There is also some mystery about the atmosphere of this planet. When
Venus passes nearly between us and the sun, her dark hemisphere is
turned towards us, her bright one being always towards the sun. But she
is not exactly on a line with the sun except on the very rare occasions
of a transit across the sun's disk. Hence, on ordinary occasions, when
she seems very near on a line with the sun, we see a very small part of
the illuminated hemisphere, which now presents the form of a very thin
crescent like the new moon. And this crescent is supposed to be a
little broader than it would be if only half the planet were
illuminated, and to encircle rather more than half the planet. Now,
this is just the effect that would be produced by an atmosphere
refracting the sun's light around the edge of the illuminated
hemisphere.

The difficulty of observations of this kind is such that the conclusion
may be open to doubt. What is seen during transits of Venus over the
sun's disk leads to more certain, but yet very puzzling, conclusions.
The writer will describe what he saw at the Cape of Good Hope during
the transit of December 5, 1882. As the dark planet impinged on the
bright sun, it of course cut out a round notch from the edge of the
sun. At first, when this notch was small, nothing could be seen of the
outline of that part of the planet which was outside the sun. But when
half the planet was on the sun, the outline of the part still off the
sun was marked by a slender arc of light. A curious fact was that this
arc did not at first span the whole outline of the planet, but only
showed at one or two points. In a few moments another part of the
outline appeared, and then another, until, at last, the arc of light
extended around the complete outline. All this seems to show that while
the planet has an atmosphere, it is not transparent like ours, but is
so filled with mist and clouds that the sun is seen through it only as
if shining in a fog.

Not many years ago the planet Mars, which is the next one outside of
us, was supposed to have a surface like that of our earth. Some parts
were of a dark greenish gray hue; these were supposed to be seas and
oceans. Other parts had a bright, warm tint; these were supposed to be
the continents. During the last twenty years much has been learned as
to how this planet looks, and the details of its surface have been
mapped by several observers, using the best telescopes under the most
favorable conditions of air and climate. And yet it must be confessed
that the result of this labor is not altogether satisfactory. It seems
certain that the so-called seas are really land and not water. When it
comes to comparing Mars with the earth, we cannot be certain of more
than a single point of resemblance. This is that during the Martian
winter a white cap, as of snow, is formed over the pole, which
partially melts away during the summer. The conclusion that there are
oceans whose evaporation forms clouds which give rise to this snow
seems plausible. But the telescope shows no clouds, and nothing to make
it certain that there is an atmosphere to sustain them. There is no
certainty that the white deposit is what we call snow; perhaps it is
not formed of water at all. The most careful studies of the surface of
this planet, under the best conditions, are those made at the Lowell
Observatory at Flagstaff, Arizona. Especially wonderful is the system
of so-called canals, first seen by Schiaparelli, but mapped in great
detail at Flagstaff. But the nature and meaning of these mysterious
lines are still to be discovered. The result is that the question of
the real nature of the surface of Mars and of what we should see around
us could we land upon it and travel over it are still among the
unsolved problems of astronomy.

If this is the case with the nearest planets that we can study, how is
it with more distant ones? Jupiter is the only one of these of the
condition of whose surface we can claim to have definite knowledge. But
even this knowledge is meagre. The substance of what we know is that
its surface is surrounded by layers of what look like dense clouds,
through which nothing can certainly be seen.

I have already spoken of the heat of the sun and its probable origin.
But the question of its heat, though the most important, is not the
only one that the sun offers us. What is the sun? When we say that it
is a very hot globe, more than a million times as large as the earth,
and hotter than any furnace that man can make, so that literally "the
elements melt with fervent heat" even at its surface, while inside they
are all vaporized, we have told the most that we know as to what the
sun really is. Of course we know a great deal about the spots, the
rotation of the sun on its axis, the materials of which it is composed,
and how its surroundings look during a total eclipse. But all this does
not answer our question. There are several mysteries which ingenious
men have tried to explain, but they cannot prove their explanations to
be correct. One is the cause and nature of the spots. Another is that
the shining surface of the sun, the "photosphere," as it is technically
called, seems so calm and quiet while forces are acting within it of a
magnitude quite beyond our conception. Flames in which our earth and
everything on it would be engulfed like a boy's marble in a
blacksmith's forge are continually shooting up to a height of tens of
thousands of miles. One would suppose that internal forces capable of
doing this would break the surface up into billows of fire a thousand
miles high; but we see nothing of the kind. The surface of the sun
seems almost as placid as a lake.

Yet another mystery is the corona of the sun. This is something we
should never have known to exist if the sun were not sometimes totally
eclipsed by the dark body of the moon. On these rare occasions the sun
is seen to be surrounded by a halo of soft, white light, sending out
rays in various directions to great distances. This halo is called the
corona, and has been most industriously studied and photographed during
nearly every total eclipse for thirty years. Thus we have learned much
about how it looks and what its shape is. It has a fibrous, woolly
structure, a little like the loose end of a much-worn hempen rope. A
certain resemblance has been seen between the form of these seeming
fibres and that of the lines in which iron filings arrange themselves
when sprinkled on paper over a magnet. It has hence been inferred that
the sun has magnetic properties, a conclusion which, in a general way,
is supported by many other facts. Yet the corona itself remains no less
an unexplained phenomenon.

[Illustration with caption: PHOTOGRAPH OF THE CORONA OF THE SUN, TAKEN
IN TRIPOLI DURING TOTAL ECLIPSE OF AUGUST 30, 1905]

A phenomenon almost as mysterious as the solar corona is the "zodiacal
light," which any one can see rising from the western horizon just
after the end of twilight on a clear winter or spring evening. The most
plausible explanation is that it is due to a cloud of small meteoric
bodies revolving round the sun. We should hardly doubt this explanation
were it not that this light has a yet more mysterious appendage,
commonly called the Gegenschein, or counter-glow. This is a patch of
light in the sky in a direction exactly opposite that of the sun. It is
so faint that it can be seen only by a practised eye under the most
favorable conditions. But it is always there. The latest suggestion is
that it is a tail of the earth, of the same kind as the tail of a comet!

We know that the motions of the heavenly bodies are predicted with
extraordinary exactness by the theory of gravitation. When one finds
that the exact path of the moon's shadow on the earth during a total
eclipse of the sun can be mapped out many years in advance, and that
the planets follow the predictions of the astronomer so closely that,
if you could see the predicted planet as a separate object, it would
look, even in a good telescope, as if it exactly fitted over the real
planet, one thinks that here at least is a branch of astronomy which is
simply perfect. And yet the worlds themselves show slight deviations in
their movements which the astronomer cannot always explain, and which
may be due to some hidden cause that, when brought to light, shall lead
to conclusions of the greatest importance to our race.

One of these deviations is in the rotation of the earth. Sometimes, for
several years at a time, it seems to revolve a little faster, and then
again a little slower. The changes are very slight; they can be
detected only by the most laborious and refined methods; yet they must
have a cause, and we should like to know what that cause is.

The moon shows a similar irregularity of motion. For half a century,
perhaps through a whole century, she will go around the earth a little
ahead of her regular rate, and then for another half-century or more
she will fall behind. The changes are very small; they would never have
been seen with the unaided eye, yet they exist. What is their cause?
Mathematicians have vainly spent years of study in trying to answer
this question.

The orbit of Mercury is found by observations to have a slight motion
which mathematicians have vainly tried to explain. For some time it was
supposed to be caused by the attraction of an unknown planet between
Mercury and the sun, and some were so sure of the existence of this
planet that they gave it a name, calling it Vulcan. But of late years
it has become reasonably certain that no planet large enough to produce
the effect observed can be there. So thoroughly has every possible
explanation been sifted out and found wanting, that some astronomers
are now inquiring whether the law of gravitation itself may not be a
little different from what has always been supposed. A very slight
deviation, indeed, would account for the facts, but cautious
astronomers want other proofs before regarding the deviation of
gravitation as an established fact.

Intelligent men have sometimes inquired how, after devoting so much
work to the study of the heavens, anything can remain for astronomers
to find out. It is a curious fact that, although they were never
learning so fast as at the present day, yet there seems to be more to
learn now than there ever was before. Great and numerous as are the
unsolved problems of our science, knowledge is now advancing into
regions which, a few years ago, seemed inaccessible. Where it will stop
none can say.




II

THE NEW PROBLEMS OF THE UNIVERSE


The achievements of the nineteenth century are still a theme of
congratulation on the part of all who compare the present state of the
world with that of one hundred years ago. And yet, if we should fancy
the most sagacious prophet, endowed with a brilliant imagination, to
have set forth in the year 1806 the problems that the century might
solve and the things which it might do, we should be surprised to see
how few of his predictions had come to pass. He might have fancied
aerial navigation and a number of other triumphs of the same class, but
he would hardly have had either steam navigation or the telegraph in
his picture. In 1856 an article appeared in Harper's Magazine depicting
some anticipated features of life in A.D. 3000. We have since made
great advances, but they bear little resemblance to what the writer
imagined. He did not dream of the telephone, but did describe much that
has not yet come to pass and probably never will.

The fact is that, much as the nineteenth century has done, its last
work was to amuse itself by setting forth more problems for this
century to solve than it has ever itself succeeded in mastering. We
should not be far wrong in saying that to-day there are more riddles in
the universe than there were before men knew that it contained anything
more than the objects they could see.

So far as mere material progress is concerned, it may be doubtful
whether anything so epoch-making as the steam-engine or the telegraph
is held in store for us by the future. But in the field of purely
scientific discovery we are finding a crowd of things of which our
philosophy did not dream even ten years ago.

The greatest riddles which the nineteenth century has bequeathed to us
relate to subjects so widely separated as the structure of the universe
and the structure of atoms of matter. We see more and more of these
structures, and we see more and more of unity everywhere, and yet new
facts difficult of explanation are being added more rapidly than old
facts are being explained.

We all know that the nineteenth century was marked by a separation of
the sciences into a vast number of specialties, to the subdivisions of
which one could see no end. But the great work of the twentieth century
will be to combine many of these specialties. The physical philosopher
of the present time is directing his thought to the demonstration of
the unity of creation. Astronomical and physical researches are now
being united in a way which is bringing the infinitely great and the
infinitely small into one field of knowledge. Ten years ago the atoms
of matter, of which it takes millions of millions to make a drop of
water, were the minutest objects with which science could imagine
itself to be concerned, Now a body of experimentalists, prominent among
whom stand Professors J. J. Thompson, Becquerel, and Roentgen, have
demonstrated the existence of objects so minute that they find their
way among and between the atoms of matter as rain-drops do among the
buildings of a city. More wonderful yet, it seems likely, although it
has not been demonstrated, that these little things, called
"corpuscles," play an important part in what is going on among the
stars. Whether this be true or not, it is certain that there do exist
in the universe emanations of some sort, producing visible effects, the
investigation of which the nineteenth century has had to bequeath to
the twentieth.

For the purpose of the navigator, the direction of the magnetic needle
is invariable in any one place, for months and even years; but when
exact scientific observations on it are made, it is found subject to
numerous slight changes. The most regular of these consists in a daily
change of its direction. It moves one way from morning until noon, and
then, late in the afternoon and during the night, turns back again to
its original pointing. The laws of this change have been carefully
studied from observations, which show that it is least at the equator
and larger as we go north into middle latitudes; but no explanation of
it resting on an indisputable basis has ever been offered.

Besides these regular changes, there are others of a very irregular
character. Every now and then the changes in the direction of the
magnet are wider and more rapid than those which occur regularly every
day. The needle may move back and forth in a way so fitful as to show
the action of some unusual exciting cause. Such movements of the needle
are commonly seen when there is a brilliant aurora. This connection
shows that a magnetic storm and an aurora must be due to the same or
some connected causes.

Those of us who are acquainted with astronomical matters know that the
number of spots on the sun goes through a regular cycle of change,
having a period of eleven years and one or two months. Now, the curious
fact is, when the number and violence of magnetic storms are recorded
and compared, it is found that they correspond to the spots on the sun,
and go through the same period of eleven years. The conclusion seems
almost inevitable: magnetic storms are due to some emanation sent out
by the sun, which arises from the same cause that produces the spots.
This emanation does not go on incessantly, but only in an occasional
way, as storms follow each other on the earth. What is it? Every
attempt to detect it has been in vain. Professor Hale, at the Yerkes
Observatory, has had in operation from time to time, for several years,
his ingenious spectroheliograph, which photographs the sun by a single
ray of the spectrum. This instrument shows that violent actions are
going on in the sun, which ordinary observation would never lead us to
suspect. But it has failed to show with certainty any peculiar
emanation at the time of a magnetic storm or anything connected with
such a storm.

A mystery which seems yet more impenetrable is associated with the
so-called new stars which blaze forth from time to time. These offer to
our sight the most astounding phenomena ever presented to the physical
philosopher. One hundred years ago such objects offered no mystery.
There was no reason to suppose that the Creator of the universe had
ceased His functions; and, continuing them, it was perfectly natural
that He should be making continual additions to the universe of stars.
But the idea that these objects are really new creations, made out of
nothing, is contrary to all our modern ideas and not in accord with the
observed facts. Granting the possibility of a really new star--if such
an object were created, it would be destined to take its place among
the other stars as a permanent member of the universe. Instead of this,
such objects invariably fade away after a few months, and are changed
into something very like an ordinary nebula. A question of transcendent
interest is that of the cause of these outbursts. It cannot be said
that science has, up to the present time, been able to offer any
suggestion not open to question. The most definite one is the collision
theory, according to which the outburst is due to the clashing together
of two stars, one or both of which might previously have been dark,
like a planet. The stars which may be actually photographed probably
exceed one hundred millions in number, and those which give too little
light to affect the photographic plate may be vastly more numerous than
those which do. Dark stars revolve around bright ones in an infinite
variety of ways, and complex systems of bodies, the members of which
powerfully attract each other, are the rule throughout the universe.
Moreover, we can set no limit to the possible number of dark or
invisible stars that may be flying through the celestial spaces. While,
therefore, we cannot regard the theory of collision as established, it
seems to be the only one yet put forth which can lay any claim to a
scientific basis. What gives most color to it is the extreme suddenness
with which the new stars, so far as has yet been observed, invariably
blaze forth. In almost every case it has been only two or three days
from the time that the existence of such an object became known until
it had attained nearly its full brightness. In fact, it would seem that
in the case of the star in Perseus, as in most other cases, the greater
part of the outburst took place within the space of twenty-four hours.
This suddenness and rapidity is exactly what would be the result of a
collision.

The most inexplicable feature of all is the rapid formation of a nebula
around this star. In the first photographs of the latter, the
appearance presented is simply that of an ordinary star. But, in the
course of three or four months, the delicate photographs taken at the
Lick Observatory showed that a nebulous light surrounded the star, and
was continually growing larger and larger. At first sight, there would
seem to be nothing extraordinary in this fact. Great masses of
intensely hot vapor, shining by their own light, would naturally be
thrown out from the star. Or, if the star had originally been
surrounded by a very rare nebulous fog or vapor, the latter would be
seen by the brilliant light emitted by the star. On this was based an
explanation offered by Kapteyn, which at first seemed very plausible.
It was that the sudden wave of light thrown out by the star when it
burst forth caused the illumination of the surrounding vapor, which,
though really at rest, would seem to expand with the velocity of light,
as the illumination reached more and more distant regions of the
nebula. This result may be made the subject of exact calculation. The
velocity of light is such as would make a circuit of the earth more
than seven times in a second. It would, therefore, go out from the star
at the rate of a million of miles in between five and six seconds. In
the lapse of one of our days, the light would have filled a sphere
around the star having a diameter more than one hundred and fifty times
the distance of the sun from the earth, and more than five times the
dimensions of the whole solar system. Continuing its course and
enlarging its sphere day after day, the sight presented to us would
have been that of a gradually expanding nebulous mass--a globe of faint
light continually increasing in size with the velocity of light.

The first sentiment the reader will feel on this subject is doubtless
one of surprise that the distance of the star should be so great as
this explanation would imply. Six months after the explosion, the globe
of light, as actually photographed, was of a size which would have been
visible to the naked eye only as a very minute object in the sky. Is it
possible that this minute object could have been thousands of times the
dimensions of our solar system?

To see how the question stands from this point of view, we must have
some idea of the possible distance of the new star. To gain this idea,
we must find some way of estimating distances in the universe. For a
reason which will soon be apparent, we begin with the greatest
structure which nature offers to the view of man. We all know that the
Milky Way is formed of countless stars, too minute to be individually
visible to the naked eye. The more powerful the telescope through which
we sweep the heavens, the greater the number of the stars that can be
seen in it. With the powerful instruments which are now in use for
photographing the sky, the number of stars brought to light must rise
into the hundreds of millions, and the greater part of these belong to
the Milky Way. The smaller the stars we count, the greater their
comparative number in the region of the Milky Way. Of the stars visible
through the telescope, more than one-half are found in the Milky Way,
which may be regarded as a girdle spanning the entire visible universe.

Of the diameter of this girdle we can say, almost with certainty, that
it must be more than a thousand times as great as the distance of the
nearest fixed star from us, and is probably two or three times greater.
According to the best judgment we can form, our solar system is situate
near the central region of the girdle, so that the latter must be
distant from us by half its diameter. It follows that if we can imagine
a gigantic pair of compasses, of which the points extend from us to
Alpha Centauri, the nearest star, we should have to measure out at
least five hundred spaces with the compass, and perhaps even one
thousand or more, to reach the region of the Milky Way.

With this we have to connect another curious fact. Of eighteen new
stars which have been observed to blaze forth during the last four
hundred years, all are in the region of the Milky Way. This seems to
show that, as a rule, they belong to the Milky Way. Accepting this very
plausible conclusion, the new star in Perseus must have been more than
five hundred times as far as the nearest fixed star. We know that it
takes light four years to reach us from Alpha Centauri. It follows that
the new star was at a distance through which light would require more
than two thousand years to travel, and quite likely a time two or three
times this. It requires only the most elementary ideas of geometry to
see that if we suppose a ray of light to shoot from a star at such a
distance in a direction perpendicular to the line of sight from us to
the star, we can compute how fast the ray would seem to us to travel.
Granting the distance to be only two thousand light years, the apparent
size of the sphere around the star which the light would fill at the
end of one year after the explosion would be that of a coin seen at a
distance of two thousand times its radius, or one thousand times its
diameter--say, a five-cent piece at the distance of sixty feet. But, as
a matter of fact, the nebulous illumination expanded with a velocity
from ten to twenty times as great as this.

The idea that the nebulosity around the new star was formed by the
illumination caused by the light of the explosion spreading out on all
sides therefore fails to satisfy us, not because the expansion of the
nebula seemed to be so slow, but because it was many times as swift as
the speed of light. Another reason for believing that it was not a mere
wave of light is offered by the fact that it did not take place
regularly in every direction from the star, but seemed to shoot off at
various angles.

Up to the present time, the speed of light has been to science, as well
as to the intelligence of our race, almost a symbol of the greatest of
possible speeds. The more carefully we reflect on the case, the more
clearly we shall see the difficulty in supposing any agency to travel
at the rate of the seeming emanations from the new star in Perseus.

As the emanation is seen spreading day after day, the reader may
inquire whether this is not an appearance due to some other cause than
the mere motion of light. May not an explosion taking place in the
centre of a star produce an effect which shall travel yet faster than
light? We can only reply that no such agency is known to science.

But is there really anything intrinsically improbable in an agency
travelling with a speed many times that of light? In considering that
there is, we may fall into an error very much like that into which our
predecessors fell in thinking it entirely out of the range of
reasonable probability that the stars should be placed at such
distances as we now know them to be.

Accepting it as a fact that agencies do exist which travel from sun to
planet and from star to star with a speed which beggars all our
previous ideas, the first question that arises is that of their nature
and mode of action. This question is, up to the present time, one which
we do not see any way of completely answering. The first difficulty is
that we have no evidence of these agents except that afforded by their
action. We see that the sun goes through a regular course of
pulsations, each requiring eleven years for completion; and we see
that, simultaneously with these, the earth's magnetism goes through a
similar course of pulsations. The connection of the two, therefore,
seems absolutely proven. But when we ask by what agency it is possible
for the sun to affect the magnetism of the earth, and when we trace the
passage of some agent between the two bodies, we find nothing to
explain the action. To all appearance, the space between the earth and
the sun is a perfect void. That electricity cannot of itself pass
through a vacuum seems to be a well-established law of physics. It is
true that electromagnetic waves, which are supposed to be of the same
nature with those of light, and which are used in wireless telegraphy,
do pass through a vacuum and may pass from the sun to the earth. But
there is no way of explaining how such waves would either produce or
affect the magnetism of the earth.

The mysterious emanations from various substances, under certain
conditions, may have an intimate relation with yet another of the
mysteries of the universe. It is a fundamental law of the universe that
when a body emits light or heat, or anything capable of being
transformed into light or heat, it can do so only by the expenditure of
force, limited in supply. The sun and stars are continually sending out
a flood of heat. They are exhausting the internal supply of something
which must be limited in extent. Whence comes the supply? How is the
heat of the sun kept up? If it were a hot body cooling off, a very few
years would suffice for it to cool off so far that its surface would
become solid and very soon cold. In recent years, the theory
universally accepted has been that the supply of heat is kept up by the
continual contraction of the sun, by mutual gravitation of its parts as
it cools off. This theory has the advantage of enabling us to
calculate, with some approximation to exactness, at what rate the sun
must be contracting in order to keep up the supply of heat which it
radiates. On this theory, it must, ten millions of years ago, have had
twice its present diameter, while less than twenty millions of years
ago it could not have existed except as an immense nebula filling the
whole solar system. We must bear in mind that this theory is the only
one which accounts for the supply of heat, even through human history.
If it be true, then the sun, earth, and solar system must be less than
twenty million years old.

Here the geologists step in and tell us that this conclusion is wholly
inadmissible. The study of the strata of the earth and of many other
geological phenomena, they assure us, makes it certain that the earth
must have existed much in its present condition for hundreds of
millions of years. During all that time there can have been no great
diminution in the supply of heat radiated by the sun.

The astronomer, in considering this argument, has to admit that he
finds a similar difficulty in connection with the stars and nebulas. It
is an impossibility to regard these objects as new; they must be as old
as the universe itself. They radiate heat and light year after year. In
all probability, they must have been doing so for millions of years.
Whence comes the supply? The geologist may well claim that until the
astronomer explains this mystery in his own domain, he cannot declare
the conclusions of geology as to the age of the earth to be wholly
inadmissible.

Now, the scientific experiments of the last two years have brought this
mystery of the celestial spaces right down into our earthly
laboratories. M. and Madame Curie have discovered the singular metal
radium, which seems to send out light, heat, and other rays
incessantly, without, so far as has yet been determined, drawing the
required energy from any outward source. As we have already pointed
out, such an emanation must come from some storehouse of energy. Is the
storehouse, then, in the medium itself, or does the latter draw it from
surrounding objects? If it does, it must abstract heat from these
objects. This question has been settled by Professor Dewar, at the
Royal Institution, London, by placing the radium in a medium next to
the coldest that art has yet produced--liquid air. The latter is
surrounded by the only yet colder medium, liquid hydrogen, so that no
heat can reach it. Under these circumstances, the radium still gives
out heat, boiling away the liquid air until the latter has entirely
disappeared. Instead of the radiation diminishing with time, it rather
seems to increase.

Called on to explain all this, science can only say that a molecular
change must be going on in the radium, to correspond to the heat it
gives out. What that change may be is still a complete mystery. It is a
mystery which we find alike in those minute specimens of the rarest of
substances under our microscopes, in the sun, and in the vast nebulous
masses in the midst of which our whole solar system would be but a
speck. The unravelling of this mystery must be the great work of
science of the twentieth century. What results shall follow for mankind
one cannot say, any more than he could have said two hundred years ago
what modern science would bring forth. Perhaps, before future
developments, all the boasted achievements of the nineteenth century
may take the modest place which we now assign to the science of the
eighteenth century--that of the infant which is to grow into a man.




III

THE STRUCTURE OF THE UNIVERSE


The questions of the extent of the universe in space and of its
duration in time, especially of its possible infinity in either space
or time, are of the highest interest both in philosophy and science.
The traditional philosophy had no means of attacking these questions
except considerations suggested by pure reason, analogy, and that
general fitness of things which was supposed to mark the order of
nature. With modern science the questions belong to the realm of fact,
and can be decided only by the results of observation and a study of
the laws to which these results may lead.

From the philosophic stand-point, a discussion of this subject which is
of such weight that in the history of thought it must be assigned a
place above all others, is that of Kant in his "Kritik." Here we find
two opposing propositions--the thesis that the universe occupies only a
finite space and is of finite duration; the antithesis that it is
infinite both as regards extent in space and duration in time. Both of
these opposing propositions are shown to admit of demonstration with
equal force, not directly, but by the methods of reductio ad absurdum.
The difficulty, discussed by Kant, was more tersely expressed by
Hamilton in pointing out that we could neither conceive of infinite
space nor of space as bounded. The methods and conclusions of modern
astronomy are, however, in no way at variance with Kant's reasoning, so
far as it extends. The fact is that the problem with which the
philosopher of Konigsberg vainly grappled is one which our science
cannot solve any more than could his logic. We may hope to gain
complete information as to everything which lies within the range of
the telescope, and to trace to its beginning every process which we can
now see going on in space. But before questions of the absolute
beginning of things, or of the boundary beyond which nothing exists,
our means of inquiry are quite powerless.

Another example of the ancient method is found in the great work of
Copernicus. It is remarkable how completely the first expounder of the
system of the world was dominated by the philosophy of his time, which
he had inherited from his predecessors. This is seen not only in the
general course of thought through the opening chapters of his work, but
among his introductory propositions. The first of these is that the
universe--mundus--as well as the earth, is spherical in form. His
arguments for the sphericity of the earth, as derived from observation,
are little more than a repetition of those of Ptolemy, and therefore
not of special interest. His proposition that the universe is spherical
is, however, not based on observation, but on considerations of the
perfection of the spherical form, the general tendency of bodies--a
drop of water, for example--to assume this form, and the sphericity of
the sun and moon. The idea retained its place in his mind, although the
fundamental conception of his system did away with the idea of the
universe having any well-defined form.

The question as attacked by modern astronomy is this: we see scattered
through space in every direction many millions of stars of various
orders of brightness and at distances so great as to defy exact
measurement, except in the case of a few of the nearest. Has this
collection of stars any well-defined boundary, or is what we see merely
that part of an infinite mass which chances to lie within the range of
our telescopes? If we were transported to the most distant star of
which we have knowledge, should we there find ourselves still
surrounded by stars on all sides, or would the space beyond be void?
Granting that, in any or every direction, there is a limit to the
universe, and that the space beyond is therefore void, what is the form
of the whole system and the distance of its boundaries? Preliminary in
some sort to these questions are the more approachable ones: Of what
sort of matter is the universe formed? and into what sort of bodies is
this matter collected?

To the ancients the celestial sphere was a reality, instead of a mere
effect of perspective, as we regard it. The stars were set on its
surface, or at least at no great distance within its crystalline mass.
Outside of it imagination placed the empyrean. When and how these
conceptions vanished from the mind of man, it would be as hard to say
as when and how Santa Claus gets transformed in the mind of the child.
They are not treated as realities by any astronomical writer from
Ptolemy down; yet, the impressions and forms of thought to which they
gave rise are well marked in Copernicus and faintly evident in Kepler.
The latter was perhaps the first to suggest that the sun might be one
of the stars; yet, from defective knowledge of the relative brightness
of the latter, he was led to the conclusion that their distances from
each other were less than the distance which separated them from the
sun. The latter he supposed to stand in the centre of a vast vacant
region within the system of stars.

For us the great collection of millions of stars which are made known
to us by the telescope, together with all the invisible bodies which
may be contained within the limits of the system, form the universe.
Here the term "universe" is perhaps objectionable because there may be
other systems than the one with which we are acquainted. The term
stellar system is, therefore, a better one by which to designate the
collection of stars in question.

It is remarkable that the first known propounder of that theory of the
form and arrangement of the system which has been most generally
accepted seems to have been a writer otherwise unknown in
science--Thomas Wright, of Durham, England. He is said to have
published a book on the theory of the universe, about 1750. It does not
appear that this work was of a very scientific character, and it was,
perhaps, too much in the nature of a speculation to excite notice in
scientific circles. One of the curious features of the history is that
it was Kant who first cited Wright's theory, pointed out its accordance
with the appearance of the Milky Way, and showed its general
reasonableness. But, at the time in question, the work of the
philosopher of Konigsberg seems to have excited no more notice among
his scientific contemporaries than that of Wright.

Kant's fame as a speculative philosopher has so eclipsed his scientific
work that the latter has but recently been appraised at its true value.
He was the originator of views which, though defective in detail,
embodied a remarkable number of the results of recent research on the
structure and form of the universe, and the changes taking place in it.
The most curious illustration of the way in which he arrived at a
correct conclusion by defective reasoning is found in his anticipation
of the modern theory of a constant retardation of the velocity with
which the earth revolves on its axis. He conceived that this effect
must result from the force exerted by the tidal wave, as moving towards
the west it strikes the eastern coasts of Asia and America. An opposite
conclusion was reached by Laplace, who showed that the effect of this
force was neutralized by forces producing the wave and acting in the
opposite direction. And yet, nearly a century later, it was shown that
while Laplace was quite correct as regards the general principles
involved, the friction of the moving water must prevent the complete
neutralization of the two opposing forces, and leave a small residual
force acting towards the west and retarding the rotation. Kant's
conclusion was established, but by an action different from that which
he supposed.

The theory of Wright and Kant, which was still further developed by
Herschel, was that our stellar system has somewhat the form of a
flattened cylinder, or perhaps that which the earth would assume if, in
consequence of more rapid rotation, the bulging out at its equator and
the flattening at its poles were carried to an extreme limit. This form
has been correctly though satirically compared to that of a grindstone.
It rests to a certain extent, but not entirely, on the idea that the
stars are scattered through space with equal thickness in every
direction, and that the appearance of the Milky Way is due to the fact
that we, situated in the centre of this flattened system, see more
stars in the direction of the circumference of the system than in that
of its poles. The argument on which the view in question rests may be
made clear in the following way.

Let us chose for our observations that hour of the night at which the
Milky Way skirts our horizon. This is nearly the case in the evenings
of May and June, though the coincidence with the horizon can never be
exact except to observers stationed near the tropics. Using the figure
of the grindstone, we at its centre will then have its circumference
around our horizon, while the axis will be nearly vertical. The points
in which the latter intersects the celestial sphere are called the
galactic poles. There will be two of these poles, the one at the hour
in question near the zenith, the other in our nadir, and therefore
invisible to us, though seen by our antipodes. Our horizon corresponds,
as it were, to the central circle of the Milky Way, which now surrounds
us on all sides in a horizontal direction, while the galactic poles are
90 degrees distant from every part of it, as every point of the horizon
is 90 degrees from the zenith.

Let us next count the number of stars visible in a powerful telescope
in the region of the heavens around the galactic pole, now our zenith,
and find the average number per square degree. This will be the
richness of the region in stars. Then we take regions nearer the
horizontal Milky Way--say that contained between 10 degrees and 20
degrees from the zenith--and, by a similar count, find its richness in
stars. We do the same for other regions, nearer and nearer to the
horizon, till we reach the galaxy itself. The result of all the counts
will be that the richness of the sky in stars is least around the
galactic pole, and increases in every direction towards the Milky Way.

Without such counts of the stars we might imagine our stellar system to
be a globular collection of stars around which the object in question
passed as a girdle; and we might take a globe with a chain passing
around it as representative of the possible figure of the stellar
system. But the actual increase in star-thickness which we have pointed
out shows us that this view is incorrect. The nature and validity of
the conclusions to be drawn can be best appreciated by a statement of
some features of this tendency of the stars to crowd towards the
galactic circle.

Most remarkable is the fact that the tendency is seen even among the
brighter stars. Without either telescope or technical knowledge, the
careful observer of the stars will notice that the most brilliant
constellations show this tendency. The glorious Orion, Canis Major
containing the brightest star in the heavens, Cassiopeia, Perseus,
Cygnus, and Lyra with its bright-blue Vega, not to mention such
constellations as the Southern Cross, all lie in or near the Milky Way.
Schiaparelli has extended the investigation to all the stars visible to
the naked eye. He laid down on planispheres the number of such stars in
each region of the heavens of 5 degrees square. Each region was then
shaded with a tint that was darker as the region was richer in stars.
The very existence of the Milky Way was ignored in this work, though
his most darkly shaded regions lie along the course of this belt. By
drawing a band around the sky so as to follow or cover his darkest
regions, we shall rediscover the course of the Milky Way without any
reference to the actual object. It is hardly necessary to add that this
result would be reached with yet greater precision if we included the
telescopic stars to any degree of magnitude--plotting them on a chart
and shading the chart in the same way. What we learn from this is that
the stellar system is not an irregular chaos; and that notwithstanding
all its minor irregularities, it may be considered as built up with
special reference to the Milky Way as a foundation.

Another feature of the tendency in question is that it is more and more
marked as we include fainter stars in our count. The galactic region is
perhaps twice as rich in stars visible to the naked eye as the rest of
the heavens. In telescopic stars to the ninth magnitude it is three or
four times as rich. In the stars found on the photographs of the sky
made at the Harvard and other observatories, and in the stargauges of
the Herschels, it is from five to ten times as rich.

Another feature showing the unity of the system is the symmetry of the
heavens on the two sides of the galactic belt Let us return to our
supposition of such a position of the celestial sphere, with respect to
the horizon, that the latter coincides with the central line of this
belt, one galactic pole being near our zenith. The celestial hemisphere
which, being above our horizon, is visible to us, is the one to which
we have hitherto directed our attention in describing the distribution
of the stars. But below our horizon is another hemisphere, that of our
antipodes, which is the counterpart of ours. The stars which it
contains are in a different part of the universe from those which we
see, and, without unity of plan, would not be subject to the same law.
But the most accurate counts of stars that have been made fail to show
any difference in their general arrangement in the two hemispheres.
They are just as thick around the south galactic poles as around the
north one. They show the same tendency to crowd towards the Milky Way
in the hemisphere invisible to us as in the hemisphere which we see.
Slight differences and irregularities, are, indeed, found in the
enumeration, but they are no greater than must necessarily arise from
the difficulty of stopping our count at a perfectly fixed magnitude.
The aim of star-counts is not to estimate the total number of stars,
for this is beyond our power, but the number visible with a given
telescope. In such work different observers have explored different
parts of the sky, and in a count of the same region by two observers we
shall find that, although they attempt to stop at the same magnitude,
each will include a great number of stars which the other omits. There
is, therefore, room for considerable difference in the numbers of stars
recorded, without there being any actual inequality between the two
hemispheres.

A corresponding similarity is found in the physical constitution of the
stars as brought out by the spectroscope. The Milky Way is extremely
rich in bluish stars, which make up a considerable majority of the
cloudlike masses there seen. But when we recede from the galaxy on one
side, we find the blue stars becoming thinner, while those having a
yellow tinge become relatively more numerous. This difference of color
also is the same on the two sides of the galactic plane. Nor can any
systematic difference be detected between the proper motions of the
stars in these two hemispheres. If the largest known proper motion is
found in the one, the second largest is in the other. Counting all the
known stars that have proper motions exceeding a given limit, we find
about as many in one hemisphere as in the other. In this respect, also,
the universe appears to be alike through its whole extent. It is the
uniformity thus prevailing through the visible universe, as far as we
can see, in two opposite directions, which inspires us with confidence
in the possibility of ultimately reaching some well-founded conclusion
as to the extent and structure of the system.

All these facts concur in supporting the view of Wright, Kant, and
Herschel as to the form of the universe. The farther out the stars
extend in any direction, the more stars we may see in that direction.
In the direction of the axis of the cylinder, the distances of the
boundary are least, so that we see fewer stars. The farther we direct
our attention towards the equatorial regions of the system, the greater
the distance from us to the boundary, and hence the more stars we see.
The fact that the increase in the number of stars seen towards the
equatorial region of the system is greater, the smaller the stars, is
the natural consequence of the fact that distant stars come within our
view in greater numbers towards the equatorial than towards the polar
regions.

Objections have been raised to the Herschelian view on the ground that
it assumes an approximately uniform distribution of the stars in space.
It has been claimed that the fact of our seeing more stars in one
direction than in another may not arise merely from our looking through
a deeper stratum, as Herschel supposed, but may as well be due to the
stars being more thinly scattered in the direction of the axis of the
system than in that of its equatorial region. The great inequalities in
the richness of neighboring regions in the Milky Way show that the
hypothesis of uniform distribution does not apply to the equatorial
region. The claim has therefore been made that there is no proof of the
system extending out any farther in the equatorial than in the polar
direction.

The consideration of this objection requires a closer inquiry as to
what we are to understand by the form of our system. We have already
pointed out the impossibility of assigning any boundary beyond which we
can say that nothing exists. And even as regards a boundary of our
stellar system, it is impossible for us to assign any exact limit
beyond which no star is visible to us. The analogy of collections of
stars seen in various parts of the heavens leads us to suppose that
there may be no well-defined form to our system, but that, as we go out
farther and farther, we shall see occasional scattered stars to,
possibly, an indefinite distance. The truth probably is that, as in
ascending a mountain, we find the trees, which may be very dense at its
base, thin out gradually as we approach the summit, where there may be
few or none, so we might find the stars to thin out could we fly to the
distant regions of space. The practical question is whether, in such a
flight, we should find this sooner by going in the direction of the
axis of our system than by directing our course towards the Milky Way.
If a point is at length reached beyond which there are but few
scattered stars, such a point would, for us, mark the boundary of our
system. From this point of view the answer does not seem to admit of
doubt. If, going in every direction, we mark the point, if any, at
which the great mass of the stars are seen behind us, the totality of
all these points will lie on a surface of the general form that
Herschel supposed.

There is still another direct indication of the finitude of our stellar
system upon which we have not touched. If this system extended out
without limit in any direction whatever, it is shown by a geometric
process which it is not necessary to explain in the present connection,
but which is of the character of mathematical demonstration, that the
heavens would, in every direction where this was true, blaze with the
light of the noonday sun. This would be very different from the
blue-black sky which we actually see on a clear night, and which, with
a reservation that we shall consider hereafter, shows that, how far
so-ever our stellar system may extend, it is not infinite. Beyond this
negative conclusion the fact does not teach us much. Vast, indeed, is
the distance to which the system might extend without the sky appearing
much brighter than it is, and we must have recourse to other
considerations in seeking for indications of a boundary, or even of a
well-marked thinning out, of stars.

If, as was formerly supposed, the stars did not greatly differ in the
amount of light emitted by each, and if their diversity of apparent
magnitude were due principally to the greater distance of the fainter
stars, then the brightness of a star would enable us to form a more or
less approximate idea of its distance. But the accumulated researches
of the past seventy years show that the stars differ so enormously in
their actual luminosity that the apparent brightness of a star affords
us only a very imperfect indication of its distance. While, in the
general average, the brighter stars must be nearer to us than the
fainter ones, it by no means follows that a very bright star, even of
the first magnitude, is among the nearer to our system. Two stars are
worthy of especial mention in this connection, Canopus and Rigel. The
first is, with the single exception of Sirius, the brightest star in
the heavens. The other is a star of the first magnitude in the
southwest corner of Orion. The most long-continued and complete
measures of parallax yet made are those carried on by Gill, at the Cape
of Good Hope, on these two and some other bright stars. The results,
published in 1901, show that neither of these bodies has any parallax
that can be measured by the most refined instrumental means known to
astronomy. In other words, the distance of these stars is immeasurably
great. The actual amount of light emitted by each is certainly
thousands and probably tens of thousands of times that of the sun.

Notwithstanding the difficulties that surround the subject, we can at
least say something of the distance of a considerable number of the
stars. Two methods are available for our estimate--measures of parallax
and determination of proper motions.

The problem of stellar parallax, simple though it is in its conception,
is the most delicate and difficult of all which the practical
astronomer has to encounter. An idea of it may be gained by supposing a
minute object on a mountain-top, we know not how many miles away, to be
visible through a telescope. The observer is allowed to change the
position of his instrument by two inches, but no more. He is required
to determine the change in the direction of the object produced by this
minute displacement with accuracy enough to determine the distance of
the mountain. This is quite analogous to the determination of the
change in the direction in which we see a star as the earth, moving
through its vast circuit, passes from one extremity of its orbit to the
other. Representing this motion on such a scale that the distance of
our planet from the sun shall be one inch, we find that the nearest
star, on the same scale, will be more than four miles away, and
scarcely one out of a million will be at a less distance than ten
miles. It is only by the most wonderful perfection both in the
heliometer, the instrument principally used for these measures, and in
methods of observation, that any displacement at all can be seen even
among the nearest stars. The parallaxes of perhaps a hundred stars have
been determined, with greater or less precision, and a few hundred more
may be near enough for measurement. All the others are immeasurably
distant; and it is only by statistical methods based on their proper
motions and their probable near approach to equality in distribution
that any idea can be gained of their distances.

To form a conception of the stellar system, we must have a unit of
measure not only exceeding any terrestrial standard, but even any
distance in the solar system. For purely astronomical purposes the most
convenient unit is the distance corresponding to a parallax of 1",
which is a little more than 200,000 times the sun's distance. But for
the purposes of all but the professional astronomer the most convenient
unit will be the light-year--that is, the distance through which light
would travel in one year. This is equal to the product of 186,000
miles, the distance travelled in one second, by 31,558,000, the number
of seconds in a year. The reader who chooses to do so may perform the
multiplication for himself. The product will amount to about 63,000
times the distance of the sun.

[Illustration with caption: A Typical Star Cluster--Centauri]

The nearest star whose distance we know, Alpha Centauri, is distant
from us more than four light-years. In all likelihood this is really
the nearest star, and it is not at all probable that any other star
lies within six light-years. Moreover, if we were transported to this
star the probability seems to be that the sun would now be the nearest
star to us. Flying to any other of the stars whose parallax has been
measured, we should probably find that the average of the six or eight
nearest stars around us ranges somewhere between five and seven
light-years. We may, in a certain sense, call eight light-years a
star-distance, meaning by this term the average of the nearest
distances from one star to the surrounding ones.

To put the result of measures of parallax into another form, let us
suppose, described around our sun as a centre, a system of concentric
spheres each of whose surfaces is at the distance of six light-years
outside the sphere next within it. The inner is at the distance of six
light-years around the sun. The surface of the second sphere will be
twelve light-years away, that of the third eighteen, etc. The volumes
of space within each of these spheres will be as the cubes of the
diameters. The most likely conclusion we can draw from measures of
parallax is that the first sphere will contain, beside the sun at its
centre, only Alpha Centauri. The second, twelve light-years away, will
probably contain, besides these two, six other stars, making eight in
all. The third may contain twenty-one more, making twenty-seven stars
within the third sphere, which is the cube of three. Within the fourth
would probably be found sixty-four stars, this being the cube of four,
and so on.

Beyond this no measures of parallax yet made will give us much
assistance. We can only infer that probably the same law holds for a
large number of spheres, though it is quite certain that it does not
hold indefinitely. For more light on the subject we must have recourse
to the proper motions. The latest words of astronomy on this subject
may be briefly summarized. As a rule, no star is at rest. Each is
moving through space with a speed which differs greatly with different
stars, but is nearly always swift, indeed, when measured by any
standard to which we are accustomed. Slow and halting, indeed, is that
star which does not make more than a mile a second. With two or three
exceptions, where the attraction of a companion comes in, the motion of
every star, so far as yet determined, takes place in a straight line.
In its outward motion the flying body deviates neither to the right nor
left. It is safe to say that, if any deviation is to take place,
thousands of years will be required for our terrestrial observers to
recognize it.

Rapid as the course of these objects is, the distances which we have
described are such that, in the great majority of cases, all the
observations yet made on the positions of the stars fail to show any
well-established motion. It is only in the case of the nearer of these
objects that we can expect any motion to be perceptible during the
period, in no case exceeding one hundred and fifty years, through which
accurate observations extend. The efforts of all the observatories
which engage in such work are, up to the present time, unequal to the
task of grappling with the motions of all the stars that can be seen
with the instruments, and reaching a decision as to the proper motion
in each particular case. As the question now stands, the aim of the
astronomer is to determine what stars have proper motions large enough
to be well established. To make our statement on this subject clear, it
must be understood that by this term the astronomer does not mean the
speed of a star in space, but its angular motion as he observes it on
the celestial sphere. A star moving forward with a given speed will
have a greater proper motion according as it is nearer to us. To avoid
all ambiguity, we shall use the term "speed" to express the velocity in
miles per second with which such a body moves through space, and the
term "proper motion" to express the apparent angular motion which the
astronomer measures upon the celestial sphere.

Up to the present time, two stars have been found whose proper motions
are so large that, if continued, the bodies would make a complete
circuit of the heavens in less than 200,000 years. One of these would
require about 160,000; the other about 180,000 years for the circuit.
Of other stars having a rapid motion only about one hundred would
complete their course in less than a million of years.

Quite recently a system of observations upon stars to the ninth
magnitude has been nearly carried through by an international
combination of observatories. The most important conclusion from these
observations relates to the distribution of the stars with reference to
the Milky Way, which we have already described. We have shown that
stars of every magnitude, bright and faint, show a tendency to crowd
towards this belt. It is, therefore, remarkable that no such tendency
is seen in the case of those stars which have proper motions large
enough to be accurately determined. So far as yet appears, such stars
are equally scattered over the heavens, without reference to the course
of the Milky Way. The conclusion is obvious. These stars are all inside
the girdle of the Milky Way, and within the sphere which contains them
the distribution in space is approximately uniform. At least there is
no well-marked condensation in the direction of the galaxy nor any
marked thinning out towards its poles. What can we say as to the extent
of this sphere?

To answer this question, we have to consider whether there is any
average or ordinary speed that a star has in space. A great number of
motions in the line of sight--that is to say, in the direction of the
line from us to the star--have been measured with great precision by
Campbell at the Lick Observatory, and by other astronomers. The
statistical investigations of Kaptoyn also throw much light on the
subject. The results of these investigators agree well in showing an
average speed in space--a straight-ahead motion we may call it--of
twenty-one miles per second. Some stars may move more slowly than this
to any extent; others more rapidly. In two or three cases the speed
exceeds one hundred miles per second, but these are quite exceptional.
By taking several thousand stars having a given proper motion, we may
form a general idea of their average distance, though a great number of
them will exceed this average to a considerable extent. The conclusion
drawn in this way would be that the stars having an apparent proper
motion of 10" per century or more are mostly contained within, or lie
not far outside of a sphere whose surface is at a distance from us of
200 light-years. Granting the volume of space which we have shown that
nature seems to allow to each star, this sphere should contain 27,000
stars in all. There are about 10,000 stars known to have so large a
proper motion as 10". But there is no actual discordance between these
results, because not only are there, in all probability, great numbers
of stars of which the proper motion is not yet recognized, but there
are within the sphere a great number of stars whose motion is less than
the average. On the other hand, it is probable that a considerable
number of the 10,000 stars lie at a distance at least one-half greater
than that of the radius of the sphere.

On the whole, it seems likely that, out to a distance of 300 or even
400 light-years, there is no marked inequality in star distribution. If
we should explore the heavens to this distance, we should neither find
the beginning of the Milky Way in one direction nor a very marked
thinning out in the other. This conclusion is quite accordant with the
probabilities of the case. If all the stars which form the groundwork
of the Milky Way should be blotted out, we should probably find
100,000,000, perhaps even more, remaining. Assigning to each star the
space already shown to be its quota, we should require a sphere of
about 3000 light-years radius to contain such a number of stars. At
some such distance as this, we might find a thinning out of the stars
in the direction of the galactic poles, or the commencement of the
Milky Way in the direction of this stream.

Even if this were not found at the distance which we have supposed, it
is quite certain that, at some greater distance, we should at least
find that the region of the Milky Way is richer in stars than the
region near the galactic poles. There is strong reason, based on the
appearance of the stars of the Milky Way, their physical constitution,
and their magnitudes as seen in the telescope, to believe that, were we
placed on one of these stars, we should find the stars around us to be
more thickly strewn than they are around our system. In other words,
the quota of space filled by each star is probably less in the region
of the Milky Way than it is near the centre where we seem to be
situated.

We are, therefore, presented with what seems to be the most
extraordinary spectacle that the universe can offer, a ring of stars
spanning it, and including within its limits by far the great majority
of the stars within our system. We have in this spectacle another
example of the unity which seems to pervade the system. We might
imagine the latter so arranged as to show diversity to any extent. We
might have agglomerations of stars like those of the Milky Way situated
in some corner of the system, or at its centre, or scattered through it
here and there in every direction. But such is not the case. There are,
indeed, a few star-clusters scattered here and there through the
system; but they are essentially different from the clusters of the
Milky Way, and cannot be regarded as forming an important part of the
general plan. In the case of the galaxy we have no such scattering, but
find the stars built, as it were, into this enormous ring, having
similar characteristics throughout nearly its whole extent, and having
within it a nearly uniform scattering of stars, with here and there
some collected into clusters. Such, to our limited vision, now appears
the universe as a whole.

We have already alluded to the conclusion that an absolutely infinite
system of stars would cause the entire heavens to be filled with a
blaze of light as bright as the sun. It is also true that the
attractive force within such a universe would be infinitely great in
some direction or another. But neither of these considerations enables
us to set a limit to the extent of our system. In two remarkable papers
by Lord Kelvin which have recently appeared, the one being an address
before the British Association at its Glasgow meeting, in 1901, are
given the results of some numerical computations pertaining to this
subject. Granting that the stars are scattered promiscuously through
space with some approach to uniformity in thickness, and are of a known
degree of brilliancy, it is easy to compute how far out the system must
extend in order that, looking up at the sky, we shall see a certain
amount of light coming from the invisible stars. Granting that, in the
general average, each star is as bright as the sun, and that their
thickness is such that within a sphere of 3300 light-years there are
1,000,000,000 stars, if we inquire how far out such a system must be
continued in order that the sky shall shine with even four per cent of
the light of the sun, we shall find the distance of its boundary so
great that millions of millions of years would be required for the
light of the outer stars to reach the centre of the system. In view of
the fact that this duration in time far exceeds what seems to be the
possible life duration of a star, so far as our knowledge of it can
extend, the mere fact that the sky does not glow with any such
brightness proves little or nothing as to the extent of the system.

We may, however, replace these purely negative considerations by
inquiring how much light we actually get from the invisible stars of
our system. Here we can make a definite statement. Mark out a small
circle in the sky 1 degree in diameter. The quantity of light which we
receive on a cloudless and moonless night from the sky within this
circle admits of actual determination. From the measures so far
available it would seem that, in the general average, this quantity of
light is not very different from that of a star of the fifth magnitude.
This is something very different from a blaze of light. A star of the
fifth magnitude is scarcely more than plainly visible to ordinary
vision. The area of the whole sky is, in round numbers, about 50,000
times that of the circle we have described. It follows that the total
quantity of light which we receive from all the stars is about equal to
that of 50,000 stars of the fifth magnitude--somewhat more than 1000 of
the first magnitude. This whole amount of light would have to be
multiplied by 90,000,000 to make a light equal to that of the sun. It
is, therefore, not at all necessary to consider how far the system must
extend in order that the heavens should blaze like the sun. Adopting
Lord Kelvin's hypothesis, we shall find that, in order that we may
receive from the stars the amount of light we have designated, this
system need not extend beyond some 5000 light-years. But this
hypothesis probably overestimates the thickness of the stars in space.
It does not seem probable that there are as many as 1,000,000,000 stars
within the sphere of 3300 light-years. Nor is it at all certain that
the light of the average star is equal to that of the sun. It is
impossible, in the present state of our knowledge, to assign any
definite value to this average. To do so is a problem similar to that
of assigning an average weight to each component of the animal
creation, from the microscopic insects which destroy our plants up to
the elephant. What we can say with a fair approximation to confidence
is that, if we could fly out in any direction to a distance of 20,000,
perhaps even of 10,000, light-years, we should find that we had left a
large fraction of our system behind us. We should see its boundary in
the direction in which we had travelled much more certainly than we see
it from our stand-point.

We should not dismiss this branch of the subject without saying that
considerations are frequently adduced by eminent authorities which tend
to impair our confidence in almost any conclusion as to the limits of
the stellar system. The main argument is based on the possibility that
light is extinguished in its passage through space; that beyond a
certain distance we cannot see a star, however bright, because its
light is entirely lost before reaching us. That there could be any loss
of light in passing through an absolute vacuum of any extent cannot be
admitted by the physicist of to-day without impairing what he considers
the fundamental principles of the vibration of light. But the
possibility that the celestial spaces are pervaded by matter which
might obstruct the passage of light is to be considered. We know that
minute meteoric particles are flying through our system in such numbers
that the earth encounters several millions of them every day, which
appear to us in the familiar phenomena of shooting-stars. If such
particles are scattered through all space, they must ultimately
obstruct the passage of light. We know little of the size of these
bodies, but, from the amount of energy contained in their light as they
are consumed in the passage through our atmosphere, it does not seem at
all likely that they are larger than grains of sand or, perhaps, minute
pebbles. They are probably vastly more numerous in the vicinity of the
sun than in the interstellar spaces, since they would naturally tend to
be collected by the sun's attraction. In fact there are some reasons
for believing that most of these bodies are the debris of comets; and
the latter are now known to belong to the solar system, and not to the
universe at large.

But whatever view we take of these possibilities, they cannot
invalidate our conclusion as to the general structure of the stellar
system as we know it. Were meteors so numerous as to cut off a large
fraction of the light from the more distant stars, we should see no
Milky Way, but the apparent thickness of the stars in every direction
would be nearly the same. The fact that so many more of these objects
are seen around the galactic belt than in the direction of its poles
shows that, whatever extinction light may suffer in going through the
greatest distances, we see nearly all that comes from stars not more
distant than the Milky Way itself.

Intimately connected with the subject we have discussed is the question
of the age of our system, if age it can be said to have. In considering
this question, the simplest hypothesis to suggest itself is that the
universe has existed forever in some such form as we now see it; that
it is a self-sustaining system, able to go on forever with only such
cycles of transformation as may repeat themselves indefinitely, and
may, therefore, have repeated themselves indefinitely in the past.
Ordinary observation does not make anything known to us which would
seem to invalidate this hypothesis. In looking upon the operations of
the universe, we may liken ourselves to a visitor to the earth from
another sphere who has to draw conclusions about the life of an
individual man from observations extending through a few days. During
that time, he would see no reason why the life of the man should have
either a beginning or an end. He sees a daily round of change, activity
and rest, nutrition and waste; but, at the end of the round, the
individual is seemingly restored to his state of the day before. Why
may not this round have been going on forever, and continue in the
future without end? It would take a profounder course of observation
and a longer time to show that, notwithstanding this seeming
restoration, an imperceptible residual of vital energy, necessary to
the continuance of life, has not been restored, and that the loss of
this residuum day by day must finally result in death.

The case is much the same with the great bodies of the universe.
Although, to superficial observation, it might seem that they could
radiate their light forever, the modern generalizations of physics show
that such cannot be the case. The radiation of light necessarily
involves a corresponding loss of heat and with it the expenditure of
some form of energy. The amount of energy within any body is
necessarily limited. The supply must be exhausted unless the energy of
the light sent out into infinite space is, in some way, restored to the
body which expended it. The possibility of such a restoration
completely transcends our science. How can the little vibration which
strikes our eye from some distant star, and which has been perhaps
thousands of years in reaching us, find its way back to its origin? The
light emitted by the sun 10,000 years ago is to-day pursuing its way in
a sphere whose surface is 10,000 light-years distant on all sides.
Science has nothing even to suggest the possibility of its restoration,
and the most delicate observations fail to show any return from the
unfathomable abyss.

Up to the time when radium was discovered, the most careful
investigations of all conceivable sources of supply had shown only one
which could possibly be of long duration. This is the contraction which
is produced in the great incandescent bodies of the universe by the
loss of the heat which they radiate. As remarked in the preceding
essay, the energy generated by the sun's contraction could not have
kept up its present supply of heat for much more than twenty or thirty
millions of years, while the study of earth and ocean shows evidence of
the action of a series of causes which must have been going on for
hundreds of millions of years.

The antagonism between the two conclusions is even more marked than
would appear from this statement. The period of the sun's heat set by
the astronomical physicist is that during which our luminary could
possibly have existed in its present form. The period set by the
geologist is not merely that of the sun's existence, but that during
which the causes effecting geological changes have not undergone any
complete revolution. If, at any time, the sun radiated much less than
its present amount of heat, no water could have existed on the earth's
surface except in the form of ice; there would have been scarcely any
evaporation, and the geological changes due to erosion could not have
taken place. Moreover, the commencement of the geological operations of
which we speak is by no means the commencement of the earth's
existence. The theories of both parties agree that, for untold aeons
before the geological changes now visible commenced, our planet was a
molten mass, perhaps even an incandescent globe like the sun. During
all those aeons the sun must have been in existence as a vast nebulous
mass, first reaching as far as the earth's orbit, and slowly
contracting its dimensions. And these aeons are to be included in any
estimate of the age of the sun.

The doctrine of cosmic evolution--the theory which in former times was
generally known as the nebular hypothesis--that the heavenly bodies
were formed by the slow contraction of heated nebulous masses, is
indicated by so many facts that it seems scarcely possible to doubt it
except on the theory that the laws of nature were, at some former time,
different from those which we now see in operation. Granting the
evolutionary hypothesis, every star has its lifetime. We can even lay
down the law by which it passes from infancy to old age. All stars do
not have the same length of life; the rule is that the larger the star,
or the greater the mass of matter which composes it, the longer will it
endure. Up to the present time, science can do nothing more than point
out these indications of a beginning, and their inevitable consequence,
that there is to be an end to the light and heat of every heavenly
body. But no cautious thinker can treat such a subject with the ease of
ordinary demonstration. The investigator may even be excused if he
stands dumb with awe before the creation of his own intellect. Our
accurate records of the operations of nature extend through only two or
three centuries, and do not reach a satisfactory standard until within
a single century. The experience of the individual is limited to a few
years, and beyond this period he must depend upon the records of his
ancestors. All his knowledge of the laws of nature is derived from this
very limited experience. How can he essay to describe what may have
been going on hundreds of millions of years in the past? Can he dare to
say that nature was the same then as now?

It is a fundamental principle of the theory of evolution, as developed
by its greatest recent expounder, that matter itself is eternal, and
that all the changes which have taken place in the universe, so far as
made up of matter, are in the nature of transformations of this eternal
substance. But we doubt whether any physical philosopher of the present
day would be satisfied to accept any demonstration of the eternity of
matter. All he would admit is that, so far as his observation goes, no
change in the quantity of matter can be produced by the action of any
known cause. It seems to be equally uncreatable and indestructible. But
he would, at the same time, admit that his experience no more sufficed
to settle the question than the observation of an animal for a single
day would settle the question of the duration of its life, or prove
that it had neither beginning nor end. He would probably admit that
even matter itself may be a product of evolution. The astronomer finds
it difficult to conceive that the great nebulous masses which he sees
in the celestial spaces--millions of times larger than the whole solar
system, yet so tenuous that they offer not the slightest obstruction to
the passage of a ray of light through their whole length--situated in
what seems to be a region of eternal cold, below anything that we can
produce on the earth's surface, yet radiating light, and with it heat,
like an incandescent body--can be made up of the same kind of substance
that we have around us on the earth's surface. Who knows but that the
radiant property that Becquerel has found in certain forms of matter
may be a residuum of some original form of energy which is inherent in
great cosmical masses, and has fed our sun during all the ages required
by the geologist for the structure of the earth's crusts? It may be
that in this phenomenon we have the key to the great riddle of the
universe, with which profounder secrets of matter than any we have
penetrated will be opened to the eyes of our successors.




IV

THE EXTENT OF THE UNIVERSE


We cannot expect that the wisest men of our remotest posterity, who can
base their conclusions upon thousands of years of accurate observation,
will reach a decision on this subject without some measure of reserve.
Such being the case, it might appear the dictate of wisdom to leave its
consideration to some future age, when it may be taken up with better
means of information than we now possess. But the question is one which
will refuse to be postponed so long as the propensity to think of the
possibilities of creation is characteristic of our race. The issue is
not whether we shall ignore the question altogether, like Eve in the
presence of Raphael; but whether in studying it we shall confine our
speculations within the limits set by sound scientific reasoning.
Essaying to do this, I invite the reader's attention to what science
may suggest, admitting in advance that the sphere of exact knowledge is
small compared with the possibilities of creation, and that outside
this sphere we can state only more or less probable conclusions.

The reader who desires to approach this subject in the most receptive
spirit should begin his study by betaking himself on a clear, moonless
evening, when he has no earthly concern to disturb the serenity of his
thoughts, to some point where he can lie on his back on bench or roof,
and scan the whole vault of heaven at one view. He can do this with the
greatest pleasure and profit in late summer or autumn--winter would do
equally well were it possible for the mind to rise so far above bodily
conditions that the question of temperature should not enter. The
thinking man who does this under circumstances most favorable for calm
thought will form a new conception of the wonder of the universe. If
summer or autumn be chosen, the stupendous arch of the Milky Way will
pass near the zenith, and the constellation Lyra, led by its beautiful
blue Vega of the first magnitude, may be not very far from that point.
South of it will be seen the constellation Aquila, marked by the bright
Altair, between two smaller but conspicuous stars. The bright Arcturus
will be somewhere in the west, and, if the observation is not made too
early in the season, Aldebaran will be seen somewhere in the east. When
attention is concentrated on the scene the thousands of stars on each
side of the Milky Way will fill the mind with the consciousness of a
stupendous and all-embracing frame, beside which all human affairs sink
into insignificance. A new idea will be formed of such a well-known
fact of astronomy as the motion of the solar system in space, by
reflecting that, during all human history, the sun, carrying the earth
with it, has been flying towards a region in or just south of the
constellation Lyra, with a speed beyond all that art can produce on
earth, without producing any change apparent to ordinary vision in the
aspect of the constellation. Not only Lyra and Aquila, but every one of
the thousand stars which form the framework of the sky, were seen by
our earliest ancestors just as we see them now. Bodily rest may be
obtained at any time by ceasing from our labors, and weary systems may
find nerve rest at any summer resort; but I know of no way in which
complete rest can be obtained for the weary soul--in which the mind can
be so entirely relieved of the burden of all human anxiety--as by the
contemplation of the spectacle presented by the starry heavens under
the conditions just described. As we make a feeble attempt to learn
what science can tell us about the structure of this starry frame, I
hope the reader will allow me to at least fancy him contemplating it in
this way.

The first question which may suggest itself to the inquiring reader is:
How is it possible by any methods of observation yet known to the
astronomer to learn anything about the universe as a whole? We may
commence by answering this question in a somewhat comprehensive way. It
is possible only because the universe, vast though it is, shows certain
characteristics of a unified and bounded whole. It is not a chaos, it
is not even a collection of things, each of which came into existence
in its own separate way. If it were, there would be nothing in common
between two widely separate regions of the universe. But, as a matter
of fact, science shows unity in the whole structure, and diversity only
in details. The Milky Way itself will be seen by the most ordinary
observer to form a single structure. This structure is, in some sort,
the foundation on which the universe is built. It is a girdle which
seems to span the whole of creation, so far as our telescopes have yet
enabled us to determine what creation is; and yet it has elements of
similarity in all its parts. What has yet more significance, it is in
some respects unlike those parts of the universe which lie without it,
and even unlike those which lie in that central region within it where
our system is now situated. The minute stars, individually far beyond
the limit of visibility to the naked eye, which form its cloudlike
agglomerations, are found to be mostly bluer in color, from one extreme
to the other, than the general average of the stars which make up the
rest of the universe.

In the preceding essay on the structure of the universe, we have
pointed out several features of the universe showing the unity of the
whole. We shall now bring together these and other features with a view
of showing their relation to the question of the extent of the universe.

The Milky Way being in a certain sense the foundation on which the
whole system is constructed, we have first to notice the symmetry of
the whole. This is seen in the fact that a certain resemblance is found
in any two opposite regions of the sky, no matter where we choose them.
If we take them in the Milky Way, the stars are more numerous than
elsewhere; if we take opposite regions in or near the Milky Way, we
shall find more stars in both of them than elsewhere; if we take them
in the region anywhere around the poles of the Milky Way, we shall find
fewer stars, but they will be equally numerous in each of the two
regions. We infer from this that whatever cause determined the number
of the stars in space was of the same nature in every two antipodal
regions of the heavens.

Another unity marked with yet more precision is seen in the chemical
elements of which stars are composed. We know that the sun is composed
of the same elements which we find on the earth and into which we
resolve compounds in our laboratories. These same elements are found in
the most distant stars. It is true that some of these bodies seem to
contain elements which we do not find on earth. But as these unknown
elements are scattered from one extreme of the universe to the other,
they only serve still further to enforce the unity which runs through
the whole. The nebulae are composed, in part at least, of forms of
matter dissimilar to any with which we are acquainted. But, different
though they may be, they are alike in their general character
throughout the whole field we are considering. Even in such a feature
as the proper motions of the stars, the same unity is seen. The reader
doubtless knows that each of these objects is flying through space on
its own course with a speed comparable with that of the earth around
the sun. These speeds range from the smallest limit up to more than one
hundred miles a second. Such diversity might seem to detract from the
unity of the whole; but when we seek to learn something definite by
taking their average, we find this average to be, so far as can yet be
determined, much the same in opposite regions of the universe. Quite
recently it has become probable that a certain class of very bright
stars known as Orion stars--because there are many of them in the most
brilliant of our constellations--which are scattered along the whole
course of the Milky Way, have one and all, in the general average,
slower motions than other stars. Here again we have a definable
characteristic extending through the universe. In drawing attention to
these points of similarity throughout the whole universe, it must not
be supposed that we base our conclusions directly upon them. The point
they bring out is that the universe is in the nature of an organized
system; and it is upon the fact of its being such a system that we are
able, by other facts, to reach conclusions as to its structure, extent,
and other characteristics.

One of the great problems connected with the universe is that of its
possible extent. How far away are the stars? One of the unities which
we have described leads at once to the conclusion that the stars must
be at very different distances from us; probably the more distant ones
are a thousand times as far as the nearest; possibly even farther than
this. This conclusion may, in the first place, be based on the fact
that the stars seem to be scattered equally throughout those regions of
the universe which are not connected with the Milky Way. To illustrate
the principle, suppose a farmer to sow a wheat-field of entirely
unknown extent with ten bushels of wheat. We visit the field and wish
to have some idea of its acreage. We may do this if we know how many
grains of wheat there are in the ten bushels. Then we examine a space
two or three feet square in any part of the field and count the number
of grains in that space. If the wheat is equally scattered over the
whole field, we find its extent by the simple rule that the size of the
field bears the same proportion to the size of the space in which the
count was made that the whole number of grains in the ten bushels sown
bears to the number of grains counted. If we find ten grains in a
square foot, we know that the number of square feet in the whole field
is one-tenth that of the number of grains sown. So it is with the
universe of stars. If the latter are sown equally through space, the
extent of the space occupied must be proportional to the number of
stars which it contains.

But this consideration does not tell us anything about the actual
distance of the stars or how thickly they may be scattered. To do this
we must be able to determine the distance of a certain number of stars,
just as we suppose the farmer to count the grains in a certain small
extent of his wheat-field. There is only one way in which we can make a
definite measure of the distance of any one star. As the earth swings
through its vast annual circuit round the sun, the direction of the
stars must appear to be a little different when seen from one extremity
of the circuit than when seen from the other. This difference is called
the parallax of the stars; and the problem of measuring it is one of
the most delicate and difficult in the whole field of practical
astronomy.

The nineteenth century was well on its way before the instruments of
the astronomer were brought to such perfection as to admit of the
measurement. From the time of Copernicus to that of Bessel many
attempts had been made to measure the parallax of the stars, and more
than once had some eager astronomer thought himself successful. But
subsequent investigation always showed that he had been mistaken, and
that what he thought was the effect of parallax was due to some other
cause, perhaps the imperfections of his instrument, perhaps the effect
of heat and cold upon it or upon the atmosphere through which he was
obliged to observe the star, or upon the going of his clock. Thus
things went on until 1837, when Bessel announced that measures with a
heliometer--the most refined instrument that has ever been used in
measurement--showed that a certain star in the constellation Cygnus had
a parallax of one-third of a second. It may be interesting to give an
idea of this quantity. Suppose one's self in a house on top of a
mountain looking out of a window one foot square, at a house on another
mountain one hundred miles away. One is allowed to look at that distant
house through one edge of the pane of glass and then through the
opposite edge; and he has to determine the change in the direction of
the distant house produced by this change of one foot in his own
position. From this he is to estimate how far off the other mountain
is. To do this, one would have to measure just about the amount of
parallax that Bessel found in his star. And yet this star is among the
few nearest to our system. The nearest star of all, Alpha Centauri,
visible only in latitudes south of our middle ones, is perhaps half as
far as Bessel's star, while Sirius and one or two others are nearly at
the same distance. About 100 stars, all told, have had their parallax
measured with a greater or less degree of probability. The work is
going on from year to year, each successive astronomer who takes it up
being able, as a general rule, to avail himself of better instruments
or to use a better method. But, after all, the distances of even some
of the 100 stars carefully measured must still remain quite doubtful.

Let us now return to the idea of dividing the space in which the
universe is situated into concentric spheres drawn at various distances
around our system as a centre. Here we shall take as our standard a
distance 400,000 times that of the sun from the earth. Regarding this
as a unit, we imagine ourselves to measure out in any direction a
distance twice as great as this--then another equal distance, making
one three times as great, and so indefinitely. We then have successive
spheres of which we take the nearer one as the unit. The total space
filled by the second sphere will be 8 times the unit; that of the third
space 27 times, and so on, as the cube of each distance. Since each
sphere includes all those within it, the volume of space between each
two spheres will be proportional to the difference of these
numbers--that is, to 1, 7, 19, etc. Comparing these volumes with the
number of stars probably within them, the general result up to the
present time is that the number of stars in any of these spheres will
be about equal to the units of volume which they comprise, when we take
for this unit the smallest and innermost of the spheres, having a
radius 400,000 times the sun's distance. We are thus enabled to form
some general idea of how thickly the stars are sown through space. We
cannot claim any numerical exactness for this idea, but in the absence
of better methods it does afford us some basis for reasoning.

Now we can carry on our computation as we supposed the farmer to
measure the extent of his wheat-field. Let us suppose that there are
125,000,000 stars in the heavens. This is an exceedingly rough
estimate, but let us make the supposition for the time being. Accepting
the view that they are nearly equally scattered throughout space, it
will follow that they must be contained within a volume equal to
125,000,000 times the sphere we have taken as our unit. We find the
distance of the surface of this sphere by extracting the cube root of
this number, which gives us 500. We may, therefore, say, as the result
of a very rough estimate, that the number of stars we have supposed
would be contained within a distance found by multiplying 400,000 times
the distance of the sun by 500; that is, that they are contained within
a region whose boundary is 200,000,000 times the distance of the sun.
This is a distance through which light would travel in about 3300 years.

It is not impossible that the number of stars is much greater than that
we have supposed. Let us grant that there are eight times as many, or
1,000,000,000. Then we should have to extend the boundary of our
universe twice as far, carrying it to a distance which light would
require 6600 years to travel.

There is another method of estimating the thickness with which stars
are sown through space, and hence the extent of the universe, the
result of which will be of interest. It is based on the proper motion
of the stars. One of the greatest triumphs of astronomy of our time has
been the measurement of the actual speed at which many of the stars are
moving to or from us in space. These measures are made with the
spectroscope. Unfortunately, they can be best made only on the brighter
stars--becoming very difficult in the case of stars not plainly visible
to the naked eye. Still the motions of several hundreds have been
measured and the number is constantly increasing.

A general result of all these measures and of other estimates may be
summed up by saying that there is a certain average speed with which
the individual stars move in space; and that this average is about
twenty miles per second. We are also able to form an estimate as to
what proportion of the stars move with each rate of speed from the
lowest up to a limit which is probably as high as 150 miles per second.
Knowing these proportions we have, by observation of the proper motions
of the stars, another method of estimating how thickly they are
scattered in space; in other words, what is the volume of space which,
on the average, contains a single star. This method gives a thickness
of the stars greater by about twenty-five per cent, than that derived
from the measures of parallax. That is to say, a sphere like the second
we have proposed, having a radius 800,000 times the distance of the
sun, and therefore a diameter 1,600,000 times this distance, would,
judging by the proper motions, have ten or twelve stars contained
within it, while the measures of parallax only show eight stars within
the sphere of this diameter having the sun as its centre. The
probabilities are in favor of the result giving the greater thickness
of the stars. But, after all, the discrepancy does not change the
general conclusion as to the limits of the visible universe. If we
cannot estimate its extent with the same certainty that we can
determine the size of the earth, we can still form a general idea of it.

The estimates we have made are based on the supposition that the stars
are equally scattered in space. We have good reason to believe that
this is true of all the stars except those of the Milky Way. But, after
all, the latter probably includes half the whole number of stars
visible with a telescope, and the question may arise whether our
results are seriously wrong from this cause. This question can best be
solved by yet another method of estimating the average distance of
certain classes of stars.

The parallaxes of which we have heretofore spoken consist in the change
in the direction of a star produced by the swing of the earth from one
side of its orbit to the other. But we have already remarked that our
solar system, with the earth as one of its bodies, has been journeying
straightforward through space during all historic times. It follows,
therefore, that we are continually changing the position from which we
view the stars, and that, if the latter were at rest, we could, by
measuring the apparent speed with which they are moving in the opposite
direction from that of the earth, determine their distance. But since
every star has its own motion, it is impossible, in any one case, to
determine how much of the apparent motion is due to the star itself,
and how much to the motion of the solar system through space. Yet, by
taking general averages among groups of stars, most of which are
probably near each other, it is possible to estimate the average
distance by this method. When an attempt is made to apply it, so as to
obtain a definite result, the astronomer finds that the data now
available for the purpose are very deficient. The proper motion of a
star can be determined only by comparing its observed position in the
heavens at two widely separate epochs. Observations of sufficient
precision for this purpose were commenced about 1750 at the Greenwich
Observatory, by Bradley, then Astronomer Royal of England. But out of
3000 stars which he determined, only a few are available for the
purpose. Even since his time, the determinations made by each
generation of astronomers have not been sufficiently complete and
systematic to furnish the material for anything like a precise
determination of the proper motions of stars. To determine a single
position of any one star involves a good deal of computation, and if we
reflect that, in order to attack the problem in question in a
satisfactory way, we should have observations of 1,000,000 of these
bodies made at intervals of at least a considerable fraction of a
century, we see what an enormous task the astronomers dealing with this
problem have before them, and how imperfect must be any determination
of the distance of the stars based on our motion through space. So far
as an estimate can be made, it seems to agree fairly well with the
results obtained by the other methods. Speaking roughly, we have
reason, from the data so far available, to believe that the stars of
the Milky Way are situated at a distance between 100,000,000 and
200,000,000 times the distance of the sun. At distances less than this
it seems likely that the stars are distributed through space with some
approach to uniformity. We may state as a general conclusion, indicated
by several methods of making the estimate, that nearly all the stars
which we can see with our telescopes are contained within a sphere not
likely to be much more than 200,000,000 times the distance of the sun.

The inquiring reader may here ask another question. Granting that all
the stars we can see are contained within this limit, may there not be
any number of stars outside the limit which are invisible only because
they are too far away to be seen?

This question may be answered quite definitely if we grant that light
from the most distant stars meets with no obstruction in reaching us.
The most conclusive answer is afforded by the measure of starlight. If
the stars extended out indefinitely, then the number of those of each
order of magnitude would be nearly four times that of the magnitude
next brighter. For example, we should have nearly four times as many
stars of the sixth magnitude as of the fifth; nearly four times as many
of the seventh as of the sixth, and so on indefinitely. Now, it is
actually found that while this ratio of increase is true for the
brighter stars, it is not so for the fainter ones, and that the
increase in the number of the latter rapidly falls off when we make
counts of the fainter telescopic stars. In fact, it has long been known
that, were the universe infinite in extent, and the stars equally
scattered through all space, the whole heavens would blaze with the
light of countless millions of distant stars separately invisible even
with the telescope.

The only way in which this conclusion can be invalidated is by the
possibility that the light of the stars is in some way extinguished or
obstructed in its passage through space. A theory to this effect was
propounded by Struve nearly a century ago, but it has since been found
that the facts as he set them forth do not justify the conclusion,
which was, in fact, rather hypothetical. The theories of modern science
converge towards the view that, in the pure ether of space, no single
ray of light can ever be lost, no matter how far it may travel. But
there is another possible cause for the extinction of light. During the
last few years discoveries of dark and therefore invisible stars have
been made by means of the spectroscope with a success which would have
been quite incredible a very few years ago, and which, even to-day,
must excite wonder and admiration. The general conclusion is that,
besides the shining stars which exist in space, there may be any number
of dark ones, forever invisible in our telescopes. May it not be that
these bodies are so numerous as to cut off the light which we would
otherwise receive from the more distant bodies of the universe? It is,
of course, impossible to answer this question in a positive way, but
the probable conclusion is a negative one. We may say with certainty
that dark stars are not so numerous as to cut off any important part of
the light from the stars of the Milky Way, because, if they did, the
latter would not be so clearly seen as it is. Since we have reason to
believe that the Milky Way comprises the more distant stars of our
system, we may feel fairly confident that not much light can be cut off
by dark bodies from the most distant region to which our telescopes can
penetrate. Up to this distance we see the stars just as they are. Even
within the limit of the universe as we understand it, it is likely that
more than one-half the stars which actually exist are too faint to be
seen by human vision, even when armed with the most powerful
telescopes. But their invisibility is due only to their distance and
the faintness of their intrinsic light, and not to any obstructing
agency.

The possibility of dark stars, therefore, does not invalidate the
general conclusions at which our survey of the subject points. The
universe, so far as we can see it, is a bounded whole. It is surrounded
by an immense girdle of stars, which, to our vision, appears as the
Milky Way. While we cannot set exact limits to its distance, we may yet
confidently say that it is bounded. It has uniformities running through
its vast extent. Could we fly out to distances equal to that of the
Milky Way, we should find comparatively few stars beyond the limits of
that girdle. It is true that we cannot set any definite limit and say
that beyond this nothing exists. What we can say is that the region
containing the visible stars has some approximation to a boundary. We
may fairly anticipate that each successive generation of astronomers,
through coming centuries, will obtain a little more light on the
subject--will be enabled to make more definite the boundaries of our
system of stars, and to draw more and more probable conclusions as to
the existence or non-existence of any object outside of it. The wise
investigator of to-day will leave to them the task of putting the
problem into a more positive shape.




V

MAKING AND USING A TELESCOPE


The impression is quite common that satisfactory views of the heavenly
bodies can be obtained only with very large telescopes, and that the
owner of a small one must stand at a great disadvantage alongside of
the fortunate possessor of a great one. This is not true to the extent
commonly supposed. Sir William Herschel would have been delighted to
view the moon through what we should now consider a very modest
instrument; and there are some objects, especially the moon, which
commonly present a more pleasing aspect through a small telescope than
through a large one. The numerous owners of small telescopes throughout
the country might find their instruments much more interesting than
they do if they only knew what objects were best suited to examination
with the means at their command. There are many others, not possessors
of telescopes, who would like to know how one can be acquired, and to
whom hints in this direction will be valuable. We shall therefore give
such information as we are able respecting the construction of a
telescope, and the more interesting celestial objects to which it may
be applied.

Whether the reader does or does not feel competent to undertake the
making of a telescope, it may be of interest to him to know how it is
done. First, as to the general principles involved, it is generally
known that the really vital parts of the telescope, which by their
combined action perform the office of magnifying the object looked at,
are two in number, the OBJECTIVE and the EYE-PIECE. The former brings
the rays of light which emanate from the object to the focus where the
image of the object is formed. The eye-piece enables the observer to
see this image to the best advantage.

The functions of the objective as well as those of the eye-piece may,
to a certain extent, each be performed by a single lens. Galileo and
his contemporaries made their telescopes in this way, because they knew
of no way in which two lenses could be made to do better than one. But
every one who has studied optics knows that white light passing through
a single lens is not all brought to the same focus, but that the blue
light will come to a focus nearer the objective than the red light.
There will, in fact, be a succession of images, blue, green, yellow,
and red, corresponding to the colors of the spectrum. It is impossible
to see these different images clearly at the same time, because each of
them will render all the others indistinct.

The achromatic object-glass, invented by Dollond, about 1750, obviates
this difficulty, and brings all the rays to nearly the same focus.
Nearly every one interested in the subject is aware that this
object-glass is composed of two lenses--a concave one of flint-glass
and a convex one of crown-glass, the latter being on the side towards
the object. This is the one vital part of the telescope, the
construction of which involves the greatest difficulty. Once in
possession of a perfect object-glass, the rest of the telescope is a
matter of little more than constructive skill which there is no
difficulty in commanding.

The construction of the object-glass requires two completely distinct
processes: the making of the rough glass, which is the work of the
glass-maker; and the grinding and polishing into shape, which is the
work of the optician. The ordinary glass of commerce will not answer
the purpose of the telescope at all, because it is not sufficiently
clear and homogeneous. OPTICAL GLASS, as it is called, must be made of
materials selected and purified with the greatest care, and worked in a
more elaborate manner than is necessary in any other kind of glass. In
the time of Dollond it was found scarcely possible to make good disks
of flint-glass more than three or four inches in diameter. Early in the
present century, Guinand, of Switzerland, invented a process by which
disks of much larger size could be produced. In conjunction with the
celebrated Fraunhofer he made disks of nine or ten inches in diameter,
which were employed by his colaborer in constructing the telescopes
which were so famous in their time. He was long supposed to be in
possession of some secret method of avoiding the difficulties which his
predecessors had met. It is now believed that this secret, if one it
was, consisted principally in the constant stirring of the molten glass
during the process of manufacture. However this may be, it is a curious
historical fact that the most successful makers of these great disks of
glass have either been of the family of Guinand, or successors, in the
management of the family firm. It was Feil, a son-in-law or near
relative, who made the glass from which Clark fabricated the lenses of
the great telescope of the Lick Observatory. His successor, Mantois, of
Paris, carried the art to a point of perfection never before
approached. The transparency and uniformity of his disks as well as the
great size to which he was able to carry them would suggest that he and
his successors have out-distanced all competitors in the process. He it
was who made the great 40-inch lens for the Yerkes Observatory.

As optical glass is now made, the material is constantly stirred with
an iron rod during all the time it is melting in the furnace, and after
it has begun to cool, until it becomes so stiff that the stirring has
to cease. It is then placed, pot and all, in the annealing furnace,
where it is kept nearly at a melting heat for three weeks or more,
according to the size of the pot. When the furnace has cooled off, the
glass is taken out, and the pot is broken from around it, leaving only
the central mass of glass. Having such a mass, there is no trouble in
breaking it up into pieces of all desirable purity, and sufficiently
large for moderate-sized telescopes. But when a great telescope of two
feet aperture or upward is to be constructed, very delicate and
laborious operations have to be undertaken. The outside of the glass
has first to be chipped off, because it is filled with impurities from
the material of the pot itself. But this is not all. Veins of unequal
density are always found extending through the interior of the mass, no
way of avoiding them having yet been discovered. They are supposed to
arise from the materials of the pot and stirring rod, which become
mixed in with the glass in consequence of the intense heat to which all
are subjected. These veins must, so far as possible, be ground or
chipped out with the greatest care. The glass is then melted again,
pressed into a flat disk, and once more put into the annealing oven. In
fact, the operation of annealing must be repeated every time the glass
is melted. When cooled, it is again examined for veins, of which great
numbers are sure to be found. The problem now is to remove these by
cutting and grinding without either breaking the glass in two or
cutting a hole through it. If the parts of the glass are once
separated, they can never be joined without producing a bad scar at the
point of junction. So long, however, as the surface is unbroken, the
interior parts of the glass can be changed in form to any extent.
Having ground out the veins as far as possible, the glass is to be
again melted, and moulded into proper shape. In this mould great care
must be taken to have no folding of the surface. Imagining the latter
to be a sort of skin enclosing the melted glass inside, it must be
raised up wherever the glass is thinnest, and the latter allowed to
slowly run together beneath it.

[Illustration with caption: THE GLASS DISK.]

If the disk is of flint, all the veins must be ground out on the first
or second trial, because after two or three mouldings the glass will
lose its transparency. A crown disk may, however, be melted a number of
times without serious injury. In many cases--perhaps the majority--the
artisan finds that after all his months of labor he cannot perfectly
clear his glass of the noxious veins, and he has to break it up into
smaller pieces. When he finally succeeds, the disk has the form of a
thin grindstone two feet or upward in diameter, according to the size
of the telescope to be made, and from two to three inches in thickness.
The glass is then ready for the optician.

[Illustration with caption: THE OPTICIAN'S TOOL.]

The first process to be performed by the optician is to grind the glass
into the shape of a lens with perfectly spherical surfaces. The convex
surface must be ground in a saucer-shaped tool of corresponding form.
It is impossible to make a tool perfectly spherical in the first place,
but success may be secured on the geometrical principle that two
surfaces cannot fit each other in all positions unless both are
perfectly spherical. The tool of the optician is a very simple affair,
being nothing more than a plate of iron somewhat larger, perhaps a
fourth, than the lens to be ground to the corresponding curvature. In
order to insure its changing to fit the glass, it is covered on the
interior with a coating of pitch from an eighth to a quarter of an inch
thick. This material is admirably adapted to the purpose because it
gives way certainly, though very slowly, to the pressure of the glass.
In order that it may have room to change its form, grooves are cut
through it in both directions, so as to leave it in the form of
squares, like those on a chess-board.

[Illustration with caption: THE OPTICIAN'S TOOL.]

It is then sprinkled over with rouge, moistened with water, and gently
warmed. The roughly ground lens is then placed upon it, and moved from
side to side. The direction of the motion is slightly changed with
every stroke, so that after a dozen or so of strokes the lines of
motion will lie in every direction on the tool. This change of
direction is most readily and easily effected by the operator slowly
walking around as he polishes, at the same time the lens is to be
slowly turned around either in the opposite direction or more rapidly
yet in the same direction, so that the strokes of the polisher shall
cross the lens in all directions. This double motion insures every part
of the lens coming into contact with every part of the polisher, and
moving over it in every direction.

Then whatever parts either of the lens or of the polisher may be too
high to form a spherical surface will be gradually worn down, thus
securing the perfect sphericity of both.

[Illustration with caption: GRINDING A LARGE LENS.]

When the polishing is done by machinery, which is the custom in Europe,
with large lenses, the polisher is slid back and forth over the lens by
means of a crank attached to a revolving wheel. The polisher is at the
same time slowly revolving around a pivot at its centre, which pivot
the crank works into, and the glass below it is slowly turned in an
opposite direction. Thus the same effect is produced as in the other
system. Those who practice this method claim that by thus using
machinery the conditions of a uniform polish for every part of the
surface can be more perfectly fulfilled than by a hand motion. The
results, however, do not support this view. No European optician will
claim to do better than the American firm of Alvan Clark & Sons in
producing uniformly good object-glasses, and this firm always does the
work by hand, moving the glass over the polisher, and not the polisher
over the glass.

Having brought both flint and crown glasses into proper figure by this
process, they are joined together, and tested by observations either
upon a star in the heavens, or some illuminated point at a little
distance on the ground. The reflection of the sun from a drop of
quicksilver, a thermometer bulb, or even a piece of broken bottle,
makes an excellent artificial star. The very best optician will always
find that on a first trial his glass is not perfect. He will find that
he has not given exactly the proper curves to secure achromatism. He
must then change the figure of one or both the glasses by polishing it
upon a tool of slightly different curvature. He may also find that
there is some spherical aberration outstanding. He must then alter his
curve so as to correct this. The correction of these little
imperfections in the figures of the lenses so as to secure perfect
vision through them is the most difficult branch of the art of the
optician, and upon his skill in practising it will depend more than
upon anything else his ultimate success and reputation. The shaping of
a pair of lenses in the way we have described is not beyond the power
of any person of ordinary mechanical ingenuity, possessing the
necessary delicacy of touch and appreciation of the problem he is
attacking. But to make a perfect objective of considerable size, which
shall satisfy all the wants of the astronomer, is an undertaking
requiring such accuracy of eyesight, and judgment in determining where
the error lies, and such skill in manipulating so as to remove the
defects, that the successful men in any one generation can be counted
on one's fingers.

In order that the telescope may finally perform satisfactorily it is
not sufficient that the lenses should both be of proper figure; they
must also both be properly centred in their cells. If either lens is
tipped aside, or slid out from its proper central line, the definition
will be injured. As this is liable to happen with almost any telescope,
we shall explain how the proper adjustment is to be made.

The easiest way to test this adjustment is to set the cell with the two
glasses of the objective in it against a wall at night, and going to a
short distance, observe the reflection in the glass of the flame of a
candle held in the hand. Three or four reflections will be seen from
the different surfaces. The observer, holding the candle before his
eye, and having his line of sight as close as possible to the flame,
must then move until the different images of the flame coincide with
each other. If he cannot bring them into coincidence, owing to
different pairs coinciding on different sides of the flame, the glasses
are not perfectly centred upon each other. When the centring is
perfect, the observer having the light in the line of the axes of the
lenses, and (if it were possible to do so) looking through the centre
of the flame, would see the three or four images all in coincidence. As
he cannot see through the flame itself, he must look first on one side
and then on the other, and see if the arrangement of the images seen in
the lenses is symmetrical. If, going to different distances, he finds
no deviation from symmetry, in this respect the adjustment is near
enough for all practical purposes.

A more artistic instrument than a simple candle is a small concave
reflector pierced through its centre, such as is used by physicians in
examining the throat.

[Illustration with caption: IMAGE OF CANDLE-FLAME IN OBJECT-GLASS.]

[Illustration with caption: TESTING ADJUSTMENT OF OBJECT-GLASS.]

Place this reflector in the prolongation of the optical axis, set the
candle so that the light from the reflector shall be shown through the
glass, and look through the opening. Images of the reflector itself
will then be seen in the object-glass, and if the adjustment is
perfect, the reflector can be moved so that they will all come into
coincidence together.

When the objective is in the tube of the telescope, it is always well
to examine this adjustment from time to time, holding the candle so
that its light shall shine through the opening perpendicularly upon the
object-glass. The observer looks upon one side of the flame, and then
upon the other, to see if the images are symmetrical in the different
positions. If in order to see them in this way the candle has to be
moved to one side of the central line of the tube, the whole objective
must be adjusted. If two images coincide in one position of the
candle-flame, and two in another position, so that they cannot all be
brought together in any position, it shows that the glasses are not
properly adjusted in their cell. It may be remarked that this last
adjustment is the proper work of the optician, since it is so difficult
that the user of the telescope cannot ordinarily effect it. But the
perpendicularity of the whole objective to the tube of the telescope is
liable to be deranged in use, and every one who uses such an instrument
should be able to rectify an error of this kind.

The question may be asked, How much of a telescope can an amateur
observer, under any circumstances, make for himself? As a general rule,
his work in this direction must be confined to the tube and the
mounting. We should not, it is true, dare to assert that any ingenious
young man, with a clear appreciation of optical principles, could not
soon learn to grind and polish an object-glass for himself by the
method we have described, and thus obtain a much better instrument than
Galileo ever had at his command. But it would be a wonderful success if
his home-made telescope was equal to the most indifferent one which can
be bought at an optician's. The objective, complete in itself, can be
purchased at prices depending upon the size.

[Footnote: The following is a rough rule for getting an idea of the
price of an achromatic objective, made to order, of the finest quality.
Take the cube of the diameter in inches, or, which is the same thing,
calculate the contents of a cubical box which would hold a sphere of
the same diameter as the clear aperture of the glass. The price of the
glass will then range from $1 to $1.75 for each cubic inch in this box.
For example, the price of a four-inch objective will probably range
from $64 to $112. Very small object-glasses of one or two inches may be
a little higher than would be given by this rule. Instruments which are
not first-class, but will answer most of the purposes of the amateur,
are much cheaper.]

[Illustration with caption: A VERY PRIMITIVE MOUNTING FOR A TELESCOPE.]

The tube for the telescope may be made of paper, by pasting a great
number of thicknesses around a long wooden cylinder. A yet better tube
is made of a simple wooden box. The best material, however, is metal,
because wood and pasteboard are liable both to get out of shape, and to
swell under the influence of moisture. Tin, if it be of sufficient
thickness, would be a very good material. The brighter it is kept, the
better. The work of fitting the objective into one end of a tin tube of
double thickness, and properly adjusting it, will probably be quite
within the powers of the ordinary amateur. The fitting of the eye-piece
into the other end of the tube will require some skill and care both on
his own part and that of his tinsmith.

Although the construction of the eye-piece is much easier than that of
the objective, since the same accuracy in adjusting the curves is not
necessary, yet the price is lower in a yet greater degree, so that the
amateur will find it better to buy than to make his eye-piece, unless
he is anxious to test his mechanical powers. For a telescope which has
no micrometer, the Huyghenian or negative eye-piece, as it is commonly
called, is the best. As made by Huyghens, it consists of two
plano-convex lenses, with their plane sides next the eye, as shown in
the figure.

[Illustration with caption: THE HUYGHENIAN EYE-PIECE.]

So far as we have yet described our telescope it is optically complete.
If it could be used as a spy-glass by simply holding it in the hand,
and pointing at the object we wish to observe, there would be little
need of any very elaborate support. But if a telescope, even of the
smallest size, is to be used with regularity, a proper "mounting" is as
essential as a good instrument. Persons unpractised in the use of such
instruments are very apt to underrate the importance of those
accessories which merely enable us to point the telescope. An idea of
what is wanted in the mounting may readily be formed if the reader will
try to look at a star with an ordinary good-sized spy-glass held in the
hand, and then imagine the difficulties he meets with multiplied by
fifty.

The smaller and cheaper telescopes, as commonly sold, are mounted on a
simple little stand, on which the instrument admits of a horizontal and
vertical motion. If one only wants to get a few glimpses of a celestial
object, this mounting will answer his purpose. But to make anything
like a study of a celestial body, the mounting must be an equatorial
one; that is, one of the axes around which the telescope moves must be
inclined so as to point towards the pole of the heavens, which is near
the polar star. This axis will then make an angle with the horizon
equal to the latitude of the place. The telescope cannot, however, be
mounted directly on this axis, but must be attached to a second one,
itself fastened to this one.

[Illustration with caption: SECTION OF THE PRIMITIVE MOUNTING. P P.
Polar axis, bearing a fork at the upper end A. Declination axis passing
through the fork E. Section of telescope tube C. Weight to balance the
tube.]

When mounted in this way, an object can be followed in its diurnal
motion from east to west by turning on the polar axis alone. But if the
greatest facility in use is required, this motion must be performed by
clock-work. A telescope with this appendage will commonly cost one
thousand dollars and upward, so that it is not usually applied to very
small ones.

We will now suppose that the reader wishes to purchase a telescope or
an object-glass for himself, and to be able to judge of its
performance. He must have the object-glass properly adjusted in its
tube, and must use the highest power; that is, the smallest eye-piece,
which he intends to use in the instrument. Of course he understands
that in looking directly at a star or a celestial object it must appear
sharp in outline and well defined. But without long practice with good
instruments, this will not give him a very definite idea. If the person
who selects the telescope is quite unpractised, it is possible that he
can make the best test by ascertaining at what distance he can read
ordinary print. To do this he should have an eye-piece magnifying about
fifty times for each inch of aperture of the telescope. For instance,
if his telescope is three inches clear aperture, then his eye-piece
should magnify one hundred and fifty times; if the aperture is four
inches, one magnifying two hundred times may be used. This magnifying
power is, as a general rule, about the highest that can be
advantageously used with any telescope. Supposing this magnifying power
to be used, this page should be legible at a distance of four feet for
every unit of magnifying power of the telescope. For example, with a
power of 100, it should be legible at a distance of 400 feet; with a
power of 200, at 800 feet, and so on. To put the condition into another
shape: if the telescope will read the print at a distance of 150 feet
for each inch of aperture with the best magnifying power, its
performance is at least not very bad. If the magnifying power is less
than would be given by this rule, the telescope should perform a little
better; for instance, a three-inch telescope with a power of 60 should
make this page legible at a distance of 300 feet, or four feet for each
unit of power.

The test applied by the optician is much more exact, and also more
easy. He points the instrument at a star, or at the reflection of the
sun's rays from a small round piece of glass or a globule of
quicksilver several hundred yards away, and ascertains whether the rays
are all brought to a focus. This is not done by simply looking at the
star, but by alternately pushing the eye-piece in beyond the point of
distinct vision and drawing it out past the point. In this way the
image of the star will appear, not as a point, but as a round disk of
light. If the telescope is perfect, this disk will appear round and of
uniform brightness in either position of the eye-piece. But if there is
any spherical aberration or differences of density in different parts
of the glass, the image will appear distorted in various ways. If the
spherical aberration is not correct, the outer rim of the disk will be
brighter than the centre when the eye-piece is pushed in, and the
centre will be the brighter when it is drawn out. If the curves of the
glass are not even all around, the image will appear oval in one or the
other position. If there are large veins of unequal density, wings or
notches will be seen on the image. If the atmosphere is steady, the
image, when the eye-piece is pushed in, will be formed of a great
number of minute rings of light. If the glass is good, these rings will
be round, unbroken, and equally bright. We present several figures
showing how these spectral images, as they are sometimes called, will
appear; first, when the eye-piece is pushed in, and secondly, when it
is drawn out, with telescopes of different qualities.

We have thus far spoken only of the refracting telescope, because it is
the kind with which an observer would naturally seek to supply himself.
At the same time there is little doubt that the construction of a
reflector of moderate size is easier than that of a corresponding
refractor. The essential part of the reflector is a slightly concave
mirror of any metal which will bear a high polish. This mirror may be
ground and polished in the same way as a lens, only the tool must be
convex.

[Illustration with caption: SPECTRAL IMAGES OF STARS; THE UPPER LINE
SHOWING HOW THEY APPEAR WITH THE EYE-PIECE PUSHED IN, THE LOWER WITH
THE EYE-PIECE DRAWN OUT.

A The telescope is all right B Spherical aberration shown by the light
and dark centre C The objective is not spherical but elliptical D The
glass not uniform--a very bad and incurable case E One side of the
objective nearer than the other. Adjust it]

Of late years it has become very common to make the mirror of glass and
to cover the reflecting face with an exceedingly thin film of silver,
which can be polished by hand in a few minutes. Such a mirror differs
from our ordinary looking-glass in that the coating of silver is put on
the front surface, so that the light does not pass through the glass.
Moreover, the coating of silver is so thin as to be almost transparent:
in fact, the sun may be seen through it by direct vision as a faint
blue object. Silvered glass reflectors made in this way are extensively
manufactured in London, and are far cheaper than refracting telescopes
of corresponding size. Their great drawback is the want of permanence
in the silver film. In the city the film will ordinarily tarnish in a
few months from the sulphurous vapors arising from gaslights and other
sources, and even in the country it is very difficult to preserve the
mirror from the contact of everything that will injure it. In
consequence, the possessor of such a telescope, if he wishes to keep it
in order, must always be prepared to resilver and repolish it. To do
this requires such careful manipulation and management of the chemicals
that it is hardly to be expected that an amateur will take the trouble
to keep his telescope in order, unless he has a taste for chemistry as
well as for astronomy.

The curiosity to see the heavenly bodies through great telescopes is so
wide-spread that we are apt to forget how much can be seen and done
with small ones. The fact is that a large proportion of the
astronomical observations of past times have been made with what we
should now regard as very small instruments, and a good deal of the
solid astronomical work of the present time is done with meridian
circles the apertures of which ordinarily range from four to eight
inches. One of the most conspicuous examples in recent times of how a
moderate-sized instrument may be utilized is afforded by the
discoveries of double stars made by Mr. S. W. Burnham, of Chicago.
Provided with a little six-inch telescope, procured at his own expense
from the Messrs. Clark, he has discovered many hundred double stars so
difficult that they had escaped the scrutiny of Maedler and the
Struves, and gained for himself one of the highest positions among the
astronomers of the day engaged in the observation of these objects. It
was with this little instrument that on Mount Hamilton,
California--afterward the site of the great Lick Observatory--he
discovered forty-eight new double stars, which had remained unnoticed
by all previous observers. First among the objects which show
beautifully through moderate instruments stands the moon. People who
want to see the moon at an observatory generally make the mistake of
looking when the moon is full, and asking to see it through the largest
telescope. Nothing can then be made out but a brilliant blaze of light,
mottled with dark spots, and crossed by irregular bright lines. The
best time to view the moon is near or before the first quarter, or when
she is from three to eight days old. The last quarter is of course
equally favorable, so far as seeing is concerned, only one must be up
after midnight to see her in that position. Seen through a three or
four inch telescope, a day or two before the first quarter, about half
an hour after sunset, and with a magnifying power between fifty and one
hundred, the moon is one of the most beautiful objects in the heavens.
Twilight softens her radiance so that the eye is not dazzled as it will
be when the sky is entirely dark. The general aspect she then presents
is that of a hemisphere of beautiful chased silver carved out in
curious round patterns with a more than human skill. If, however, one
wishes to see the minute details of the lunar surface, in which many of
our astronomers are now so deeply interested, he must use a higher
magnifying power. The general beautiful effect is then lessened, but
more details are seen. Still, it is hardly necessary to seek for a very
large telescope for any investigation of the lunar surface. I very much
doubt whether any one has ever seen anything on the moon which could
not be made out in a clear, steady atmosphere with a six-inch telescope
of the first class.

Next to the moon, Saturn is among the most beautiful of celestial
objects. Its aspect, however, varies with its position in its orbit.
Twice in the course of a revolution, which occupies nearly thirty
years, the rings are seen edgewise, and for a few days are invisible
even in a powerful telescope. For an entire year their form may be
difficult to make out with a small telescope. These unfavorable
conditions occur in 1907 and 1921. Between these dates, especially for
some years after 1910, the position of the planet in the sky will be
the most favorable, being in northern declination, near its perihelion,
and having its rings widely open. We all know that Saturn is plainly
visible to the naked eye, shining almost like a star of the first
magnitude, so that there is no difficulty in finding it if one knows
when and where to look. In 1906-1908 its oppositions occur in the month
of September. In subsequent years, it will occur a month later every
two and a half years. The ring can be seen with a common, good
spy-glass fastened to a post so as to be steady. A four or five-inch
telescope will show most of the satellites, the division in the ring,
and, when the ring is well opened, the curious dusky ring discovered by
Bond. This "crape ring," as it is commonly called, is one of the most
singular phenomena presented by that planet.

It might be interesting to the amateur astronomer with a keen eye and a
telescope of four inches aperture or upward to frequently scrutinize
Saturn, with a view of detecting any extraordinary eruptions upon his
surface, like that seen by Professor Hall in 1876. On December 7th of
that year a bright spot was seen upon Saturn's equator. It elongated
itself from day to day, and remained visible for several weeks. Such a
thing had never before been known upon this planet, and had it not been
that Professor Hall was engaged in observations upon the satellites, it
would not have been seen then. A similar spot on the planet was
recorded in 1902, and much more extensively noticed. On this occasion
the spot appeared in a higher latitude from the planet's equator than
did Professor Hall's. At this appearance the time of the planet's
revolution on its axis was found to be somewhat greater than in 1876,
in accordance with the general law exhibited in the rotations of the
sun and of Jupiter. Notwithstanding their transient character, these
two spots have afforded the only determination of the time of
revolution of Saturn which has been made since Herschel the elder.

[Illustration with caption: THE GREAT REFRACTOR OF THE NATIONAL
OBSERVATORY AT WASHINGTON]

Of the satellites of Saturn the brightest is Titan, which can be seen
with the smallest telescope, and revolves around the planet in fifteen
days. Iapetus, the outer satellite, is remarkable for varying greatly
in brilliancy during its revolution around the planet. Any one having
the means and ability to make accurate photometrical estimates of the
light of this satellite in all points of its orbit, can thereby render
a valuable service to astronomy.

The observations of Venus, by which the astronomers of the last century
supposed themselves to have discovered its time of rotation on its
axis, were made with telescopes much inferior to ours. Although their
observations have not been confirmed, some astronomers are still
inclined to think that their results have not been refuted by the
failure of recent observers to detect those changes which the older
ones describe on the surface of the planet. With a six-inch telescope
of the best quality, and with time to choose the most favorable moment,
one will be as well equipped to settle the question of the rotation of
Venus as the best observer. The few days near each inferior conjunction
are especially to be taken advantage of.

The questions to be settled are two: first, are there any dark spots or
other markings on the disk? second, are there any irregularities in the
form of the sharp cusps? The central portions of the disk are much
darker than the outline, and it is probably this fact which has given
rise to the impression of dark spots. Unless this apparent darkness
changes from time to time, or shows some irregularity in its outline,
it cannot indicate any rotation of the planet. The best time to
scrutinize the sharp cusps will be when the planet is nearly on the
line from the earth to the sun. The best hour of the day is near
sunset, the half-hour following sunset being the best of all. But if
Venus is near the sun, she will after sunset be too low down to be well
seen, and must be looked at late in the afternoon.

The planet Mars must always be an object of great interest, because of
all the heavenly bodies it is that which appears to bear the greatest
resemblance to the earth. It comes into opposition at intervals of a
little more than two years, and can be well seen only for a month or
two before and after each opposition. It is hopeless to look for the
satellites of Mars with any but the greatest telescopes of the world.
But the markings on the surface, from which the time of rotation has
been determined, and which indicate a resemblance to the surface of our
own planet, can be well seen with telescopes of six inches aperture and
upward. One or both of the bright polar spots, which are supposed to be
due to deposits of snow, can be seen with smaller telescopes when the
situation of the planet is favorable.

The case is different with the so-called canals discovered by
Schiaparelli in 1877, which have ever since excited so much interest,
and given rise to so much discussion as to their nature. The astronomer
who has had the best opportunities for studying them is Mr. Percival
Lowell, whose observatory at Flaggstaff, Arizona, is finely situated
for the purpose, while he also has one of the best if not the largest
of telescopes. There the canals are seen as fine dark lines; but, even
then, they must be fifty miles in breadth, so that the word "canal" may
be regarded as a misnomer.

Although the planet Jupiter does not present such striking features as
Saturn, it is of even more interest to the amateur astronomer, because
he can study it with less optical power, and see more of the changes
upon its surface. Every work on astronomy tells in a general way of the
belts of Jupiter, and many speculate upon their causes. The reader of
recent works knows that Jupiter is supposed to be not a solid mass like
the earth, but a great globe of molten and vaporous matter,
intermediate in constitution between the earth and the sun. The outer
surface which we see is probably a hot mass of vapor hundreds of miles
deep, thrown up from the heated interior. The belts are probably
cloudlike forms in this vaporous mass. Certain it is that they are
continually changing, so that the planet seldom looks exactly the same
on two successive evenings. The rotation of the planet can be very well
seen by an hour's watching. In two hours an object at the centre of the
disk will move off to near the margin.

The satellites of this planet, in their ever-varying phases, are
objects of perennial interest. Their eclipses may be observed with a
very small telescope, if one knows when to look for them. To do this
successfully, and without waste of time, it is necessary to have an
astronomical ephemeris for the year. All the observable phenomena are
there predicted for the convenience of observers. Perhaps the most
curious observation to be made is that of the shadow of the satellite
crossing the disk of Jupiter. The writer has seen this perfectly with a
six-inch telescope, and a much smaller one would probably show it well.
With a telescope of this size, or a little larger, the satellites can
be seen between us and Jupiter. Sometimes they appear a little brighter
than the planet, and sometimes a little fainter.

Of the remaining large planets, Mercury, the inner one, and Uranus and
Neptune, the two outer ones, are of less interest than the others to an
amateur with a small telescope, because they are more difficult to see.
Mercury can, indeed, be observed with the smallest instrument, but no
physical configurations or changes have ever been made out upon his
surface. The question whether any such can be observed is still an open
one, which can be settled only by long and careful scrutiny. A small
telescope is almost as good for this purpose as a large one, because
the atmospheric difficulties in the way of getting a good view of the
planet cannot be lessened by an increase of telescopic power.

Uranus and Neptune are so distant that telescopes of considerable size
and high magnifying power are necessary to show their disks. In small
telescopes they have the appearance of stars, and the observer has no
way of distinguishing them from the surrounding stars unless he can
command the best astronomical appliances, such as star maps, circles on
his instrument, etc. It is, however, to be remarked, as a fact not
generally known, that Uranus can be well seen with the naked eye if one
knows where to look for it. To recognize it, it is necessary to have an
astronomical ephemeris showing its right ascension and declination, and
star maps showing where the parallels of right ascension and
declination lie among the stars. When once found by the naked eye,
there will, of course, be no difficulty in pointing the telescope upon
it.

Of celestial objects which it is well to keep a watch upon, and which
can be seen to good advantage with inexpensive instruments, the sun may
be considered as holding the first place. Astronomers who make a
specialty of solar physics have, especially in this country, so many
other duties, and their view is so often interrupted by clouds, that a
continuous record of the spots on the sun and the changes they undergo
is hardly possible. Perhaps one of the most interesting and useful
pieces of astronomical work which an amateur can perform will consist
of a record of the origin and changes of form of the solar spots and
faculae. What does a spot look like when it first comes into sight?
Does it immediately burst forth with considerable magnitude, or does it
begin as the smallest visible speck, and gradually grow? When several
spots coalesce into one, how do they do it? When a spot breaks up into
several pieces, what is the seeming nature of the process? How do the
groups of brilliant points called faculae come, change, and grow? All
these questions must no doubt be answered in various ways, according to
the behavior of the particular spot, but the record is rather meagre,
and the conscientious and industrious amateur will be able to amuse
himself by adding to it, and possibly may make valuable contributions
to science in the same way.

Still another branch of astronomical observation, in which industry and
skill count for more than expensive instruments, is the search for new
comets. This requires a very practised eye, in order that the comet may
be caught among the crowd of stars which flit across the field of view
as the telescope is moved. It is also necessary to be well acquainted
with a number of nebulae which look very much like comets. The search
can be made with almost any small telescope, if one is careful to use a
very low power. With a four-inch telescope a power not exceeding twenty
should be employed. To search with ease, and in the best manner, the
observer should have what among astronomers is familiarly known as a
"broken-backed telescope." This instrument has the eye-piece on the end
of the axis, where one would never think of looking for it. By turning
the instrument on this axis, it sweeps from one horizon through the
zenith and over to the other horizon without the observer having to
move his head. This is effected by having a reflector in the central
part of the instrument, which throws the rays of light at right angles
through the axis.

[Illustration: THE "BROKEN-BACKED COMET-SEEKER"]

How well this search can be conducted by observers with limited means
at their disposal is shown by the success of several American
observers, among whom Messrs. W. R. Brooks, E. E. Barnard, and Lewis
Swift are well known. The cometary discoveries of these men afford an
excellent illustration of how much can be done with the smallest means
when one sets to work in the right spirit.

The larger number of wonderful telescopic objects are to be sought for
far beyond the confines of the solar system, in regions from which
light requires years to reach us. On account of their great distance,
these objects generally require the most powerful telescopes to be seen
in the best manner; but there are quite a number within the range of
the amateur. Looking at the Milky Way, especially its southern part, on
a clear winter or summer evening, tufts of light will be seen here and
there. On examining these tufts with a telescope, they will be found to
consist of congeries of stars. Many of these groups are of the greatest
beauty, with only a moderate optical power. Of all the groups in the
Milky Way the best known is that in the sword-handle of Perseus, which
may be seen during the greater part of the year, and is distinctly
visible to the naked eye as a patch of diffused light. With the
telescope there are seen in this patch two closely connected clusters
of stars, or perhaps we ought rather to say two centres of condensation.

Another object of the same class is Proesepe in the constellation
Cancer. This can be very distinctly seen by the naked eye on a clear
moonless night in winter or spring as a faint nebulous object,
surrounded by three small stars. The smallest telescope shows it as a
group of stars.

Of all stellar objects, the great nebula of Orion is that which has
most fascinated the astronomers of two centuries. It is distinctly
visible to the naked eye, and may be found without difficulty on any
winter night. The three bright stars forming the sword-belt of Orion
are known to every one who has noticed that constellation. Below this
belt is seen another triplet of stars, not so bright, and lying in a
north and south direction. The middle star of this triplet is the great
nebula. At first the naked eye sees nothing to distinguish it from
other stars, but if closely scanned it will be seen to have a hazy
aspect. A four-inch telescope will show its curious form. Not the least
interesting of its features are the four stars known as the
"Trapezium," which are located in a dark region near its centre. In
fact, the whole nebula is dotted with stars, which add greatly to the
effect produced by its mysterious aspect.

The great nebula of Andromeda is second only to that of Orion in
interest. Like the former, it is distinctly visible to the naked eye,
having the aspect of a faint comet. The most curious feature of this
object is that although the most powerful telescopes do not resolve it
into stars, it appears in the spectroscope as if it were solid matter
shining by its own light.

The above are merely selections from the countless number of objects
which the heavens offer to telescopic study. Many such are described in
astronomical works, but the amateur can gratify his curiosity to almost
any extent by searching them out for himself.

[Illustration with caption: NEBULA IN ORION]

Ever since 1878 a red spot, unlike any before noticed, has generally
been visible on Jupiter. At first it was for several years a very
conspicuous object, but gradually faded away, so that since 1890 it has
been made out only with difficulty. But it is now regarded as a
permanent feature of the planet. There is some reason to believe it was
occasionally seen long before attention was first attracted to it.
Doubtless, when it can be seen at all, practice in observing such
objects is more important than size of telescope.




VI

WHAT THE ASTRONOMERS ARE DOING


In no field of science has human knowledge been more extended in our
time than in that of astronomy. Forty years ago astronomical research
seemed quite barren of results of great interest or value to our race.
The observers of the world were working on a traditional system,
grinding out results in an endless course, without seeing any prospect
of the great generalizations to which they might ultimately lead. Now
this is all changed. A new instrument, the spectroscope, has been
developed, the extent of whose revelations we are just beginning to
learn, although it has been more than thirty years in use. The
application of photography has been so extended that, in some important
branches of astronomical work, the observer simply photographs the
phenomenon which he is to study, and then makes his observation on the
developed negative.

The world of astronomy is one of the busiest that can be found to-day,
and the writer proposes, with the reader's courteous consent, to take
him on a stroll through it and see what is going on. We may begin our
inspection with a body which is, for us, next to the earth, the most
important in the universe. I mean the sun. At the Greenwich Observatory
the sun has for more than twenty years been regularly photographed on
every clear day, with the view of determining the changes going on in
its spots. In recent years these observations have been supplemented by
others, made at stations in India and Mauritius, so that by the
combination of all it is quite exceptional to have an entire day pass
without at least one photograph being taken. On these observations must
mainly rest our knowledge of the curious cycle of change in the solar
spots, which goes through a period of about eleven years, but of which
no one has as yet been able to establish the cause.

This Greenwich system has been extended and improved by an American.
Professor George E. Hale, formerly Director of the Yerkes Observatory,
has devised an instrument for taking photographs of the sun by a single
ray of the spectrum. The light emitted by calcium, the base of lime,
and one of the substances most abundant in the sun, is often selected
to impress the plate.

The Carnegie Institution has recently organized an enterprise for
carrying on the study of the sun under a combination of better
conditions than were ever before enjoyed. The first requirement in such
a case is the ablest and most enthusiastic worker in the field, ready
to devote all his energies to its cultivation. This requirement is
found in the person of Professor Hale himself. The next requirement is
an atmosphere of the greatest transparency, and a situation at a high
elevation above sea-level, so that the passage of light from the sun to
the observer shall be obstructed as little as possible by the mists and
vapors near the earth's surface. This requirement is reached by placing
the observatory on Mount Wilson, near Pasadena, California, where the
climate is found to be the best of any in the United States, and
probably not exceeded by that of any other attainable point in the
world. The third requirement is the best of instruments, specially
devised to meet the requirements. In this respect we may be sure that
nothing attainable by human ingenuity will be found wanting.

Thus provided, Professor Hale has entered upon the task of studying the
sun, and recording from day to day all the changes going on in it,
using specially devised instruments for each purpose in view.
Photography is made use of through almost the entire investigation. A
full description of the work would require an enumeration of technical
details, into which we need not enter at present. Let it, therefore,
suffice to say in a general way that the study of the sun is being
carried on on a scale, and with an energy worthy of the most important
subject that presents itself to the astronomer. Closely associated with
this work is that of Professor Langley and Dr. Abbot, at the
Astro-Physical Observatory of the Smithsonian Institution, who have
recently completed one of the most important works ever carried out on
the light of the sun. They have for years been analyzing those of its
rays which, although entirely invisible to our eyes, are of the same
nature as those of light, and are felt by us as heat. To do this,
Langley invented a sort of artificial eye, which he called a bolometer,
in which the optic nerve is made of an extremely thin strip of metal,
so slight that one can hardly see it, which is traversed by an electric
current. This eye would be so dazzled by the heat radiated from one's
body that, when in use, it must be protected from all such heat by
being enclosed in a case kept at a constant temperature by being
immersed in water. With this eye the two observers have mapped the heat
rays of the sun down to an extent and with a precision which were
before entirely unknown.

The question of possible changes in the sun's radiation, and of the
relation of those changes to human welfare, still eludes our scrutiny.
With all the efforts that have been made, the physicist of to-day has
not yet been able to make anything like an exact determination of the
total amount of heat received from the sun. The largest measurements
are almost double the smallest. This is partly due to the atmosphere
absorbing an unknown and variable fraction of the sun's rays which pass
through it, and partly to the difficulty of distinguishing the heat
radiated by the sun from that radiated by terrestrial objects.

In one recent instance, a change in the sun's radiation has been
noticed in various parts of the world, and is of especial interest
because there seems to be little doubt as to its origin. In the latter
part of 1902 an extraordinary diminution was found in the intensity of
the sun's heat, as measured by the bolometer and other instruments.
This continued through the first part of 1903, with wide variations at
different places, and it was more than a year after the first
diminution before the sun's rays again assumed their ordinary intensity.

This result is now attributed to the eruption of Mount Pelee, during
which an enormous mass of volcanic dust and vapor was projected into
the higher regions of the air, and gradually carried over the entire
earth by winds and currents. Many of our readers may remember that
something yet more striking occurred after the great cataclasm at
Krakatoa in 1883, when, for more than a year, red sunsets and red
twilights of a depth of shade never before observed were seen in every
part of the world.

What we call universology--the knowledge of the structure and extent of
the universe--must begin with a study of the starry heavens as we see
them. There are perhaps one hundred million stars in the sky within the
reach of telescopic vision. This number is too great to allow of all
the stars being studied individually; yet, to form the basis for any
conclusion, we must know the positions and arrangement of as many of
them as we can determine.

To do this the first want is a catalogue giving very precise positions
of as many of the brighter stars as possible. The principal national
observatories, as well as some others, are engaged in supplying this
want. Up to the present time about 200,000 stars visible in our
latitudes have been catalogued on this precise plan, and the work is
still going on. In that part of the sky which we never see, because it
is only visible from the southern hemisphere, the corresponding work is
far from being as extensive. Sir David Gill, astronomer at the Cape of
Good Hope, and also the directors of other southern observatories, are
engaged in pushing it forward as rapidly as the limited facilities at
their disposal will allow.

Next in order comes the work of simply listing as many stars as
possible. Here the most exact positions are not required. It is only
necessary to lay down the position of each star with sufficient
exactness to distinguish it from all its neighbors. About 400,000 stars
were during the last half-century listed in this way at the observatory
of Bonn by Argelander, Schonfeld, and their assistants. This work is
now being carried through the southern hemisphere on a large scale by
Thome, Director of the Cordoba Observatory, in the Argentine Republic.
This was founded thirty years ago by our Dr. B. A. Gould, who turned it
over to Dr. Thome in 1886. The latter has, up to the present time,
fixed and published the positions of nearly half a million stars. This
work of Thome extends to fainter stars than any other yet attempted, so
that, as it goes on, we have more stars listed in a region invisible in
middle northern latitudes than we have for that part of the sky we can
see. Up to the present time three quarto volumes giving the positions
and magnitudes of the stars have appeared. Two or three volumes more,
and, perhaps, ten or fifteen years, will be required to complete the
work.

About twenty years ago it was discovered that, by means of a telescope
especially adapted to this purpose, it was possible to photograph many
more stars than an instrument of the same size would show to the eye.
This discovery was soon applied in various quarters. Sir David Gill,
with characteristic energy, photographed the stars of the southern sky
to the number of nearly half a million. As it was beyond his power to
measure off and compute the positions of the stars from his plates, the
latter were sent to Professor J. C. Kapteyn, of Holland, who undertook
the enormous labor of collecting them into a catalogue, the last volume
of which was published in 1899. One curious result of this enterprise
is that the work of listing the stars is more complete for the southern
hemisphere than for the northern.

Another great photographic work now in progress has to do with the
millions of stars which it is impossible to handle individually.
Fifteen years ago an association of observatories in both hemispheres
undertook to make a photographic chart of the sky on the largest scale.
Some portions of this work are now approaching completion, but in
others it is still in a backward state, owing to the failure of several
South American observatories to carry out their part of the programme.
When it is all done we shall have a picture of the sky, the study of
which may require the labor of a whole generation of astronomers.

Quite independently of this work, the Harvard University, under the
direction of Professor Pickering, keeps up the work of photographing
the sky on a surprising scale. On this plan we do not have to leave it
to posterity to learn whether there is any change in the heavens, for
one result of the enterprise has been the discovery of thirteen of the
new stars which now and then blaze out in the heavens at points where
none were before known. Professor Pickering's work has been continually
enlarged and improved until about 150,000 photographic plates, showing
from time to time the places of countless millions of stars among their
fellows are now stored at the Harvard Observatory. Not less remarkable
than this wealth of material has been the development of skill in
working it up. Some idea of the work will be obtained by reflecting
that, thirty years ago, careful study of the heavens by astronomers
devoting their lives to the task had resulted in the discovery of some
two or three hundred stars, varying in their light. Now, at Harvard,
through keen eyes studying and comparing successive photographs not
only of isolated stars, but of clusters and agglomerations of stars in
the Milky Way and elsewhere, discoveries of such objects numbering
hundreds have been made, and the work is going on with ever-increasing
speed. Indeed, the number of variable stars now known is such that
their study as individual objects no longer suffices, and they must
hereafter be treated statistically with reference to their distribution
in space, and their relations to one another, as a census classifies
the entire population without taking any account of individuals.

The works just mentioned are concerned with the stars. But the heavenly
spaces contain nebulae as well as stars; and photography can now be
even more successful in picturing them than the stars. A few years ago
the late lamented Keeler, at the Lick Observatory, undertook to see
what could be done by pointing the Crossley reflecting telescope at the
sky and putting a sensitive photographic plate in the focus. He was
surprised to find that a great number of nebulae, the existence of
which had never before been suspected, were impressed on the plate. Up
to the present time the positions of about 8000 of these objects have
been listed. Keeler found that there were probably 200,000 nebulae in
the heavens capable of being photographed with the Crossley reflector.
But the work of taking these photographs is so great, and the number of
reflecting telescopes which can be applied to it so small, that no one
has ventured to seriously commence it. It is worthy of remark that only
a very small fraction of these objects which can be photographed are
visible to the eye, even with the most powerful telescope.

This demonstration of what the reflecting telescope can do may be
regarded as one of the most important discoveries of our time as to the
capabilities of astronomical instruments. It has long been known that
the image formed in the focus of the best refracting telescope is
affected by an imperfection arising from the different action of the
glasses on rays of light of different colors. Hence, the image of a
star can never be seen or photographed with such an instrument, as an
actual point, but only as a small, diffused mass. This difficulty is
avoided in the reflecting telescope; but a new difficulty is found in
the bending of the mirror under the influence of its own weight.
Devices for overcoming this had been so far from successful that, when
Mr. Crossley presented his instrument to the Lick Observatory, it was
feared that little of importance could be done with it. But as often
happens in human affairs outside the field of astronomy, when ingenious
and able men devote their attention to the careful study of a problem,
it was found that new results could be reached. Thus it was that,
before a great while, what was supposed to be an inferior instrument
proved not only to have qualities not before suspected, but to be the
means of making an important addition to the methods of astronomical
investigation.

In order that our knowledge of the position of a star may be complete,
we must know its distance. This can be measured only through the star's
parallax--that is to say, the slight change in its direction produced
by the swing of our earth around its orbit. But so vast is the distance
in question that this change is immeasurably small, except for,
perhaps, a few hundred stars, and even for these few its measurement
almost baffles the skill of the most expert astronomer. Progress in
this direction is therefore very slow, and there are probably not yet a
hundred stars of which the parallax has been ascertained with any
approach to certainty. Dr. Chase is now completing an important work of
this kind at the Yale Observatory.

To the most refined telescopic observations, as well as to the naked
eye, the stars seem all alike, except that they differ greatly in
brightness, and somewhat in color. But when their light is analyzed by
the spectroscope, it is found that scarcely any two are exactly alike.
An important part of the work of the astro-physical observatories,
especially that of Harvard, consists in photographing the spectra of
thousands of stars, and studying the peculiarities thus brought out. At
Harvard a large portion of this work is done as part of the work of the
Henry Draper Memorial, established by his widow in memory of the
eminent investigator of New York, who died twenty years ago.

By a comparison of the spectra of stars Sir William Huggins has
developed the idea that these bodies, like human beings, have a life
history. They are nebulae in infancy, while the progress to old age is
marked by a constant increase in the density of their substance. Their
temperature also changes in a way analogous to the vigor of the human
being. During a certain time the star continually grows hotter and
hotter. But an end to this must come, and it cools off in old age. What
the age of a star may be is hard even to guess. It is many millions of
years, perhaps hundreds, possibly even thousands, of millions.

Some attempt at giving the magnitude is included in every considerable
list of stars. The work of determining the magnitudes with the greatest
precision is so laborious that it must go on rather slowly. It is being
pursued on a large scale at the Harvard Observatory, as well as in that
of Potsdam, Germany.

We come now to the question of changes in the appearance of bright
stars. It seems pretty certain that more than one per cent of these
bodies fluctuate to a greater or less extent in their light.
Observations of these fluctuations, in the case of at least the
brighter stars, may be carried on without any instrument more expensive
than a good opera-glass--in fact, in the case of stars visible to the
naked eye, with no instrument at all.

As a general rule, the light of these stars goes through its changes in
a regular period, which is sometimes as short as a few hours, but
generally several days, frequently a large fraction of a year or even
eighteen months. Observations of these stars are made to determine the
length of the period and the law of variation of the brightness. Any
person with a good eye and skill in making estimates can make the
observations if he will devote sufficient pains to training himself;
but they require a degree of care and assiduity which is not to be
expected of any one but an enthusiast on the subject. One of the most
successful observers of the present time is Mr. W. A. Roberts, a
resident of South Africa, whom the Boer war did not prevent from
keeping up a watch of the southern sky, which has resulted in greatly
increasing our knowledge of variable stars. There are also quite a
number of astronomers in Europe and America who make this particular
study their specialty.

During the past fifteen years the art of measuring the speed with which
a star is approaching us or receding from us has been brought to a
wonderful degree of perfection. The instrument with which this was
first done was the spectroscope; it is now replaced with another of the
same general kind, called the spectrograph. The latter differs from the
other only in that the spectrum of the star is photographed, and the
observer makes his measures on the negative. This method was first
extensively applied at the Potsdam Observatory in Germany, and has
lately become one of the specialties of the Lick Observatory, where
Professor Campbell has brought it to its present degree of perfection.
The Yerkes Observatory is also beginning work in the same line, where
Professor Frost is already rivalling the Lick Observatory in the
precision of his measures.

Let us now go back to our own little colony and see what is being done
to advance our knowledge of the solar system. This consists of planets,
on one of which we dwell, moons revolving around them, comets, and
meteoric bodies. The principal national observatories keep up a more or
less orderly system of observations of the positions of the planets and
their satellites in order to determine the laws of their motion. As in
the case of the stars, it is necessary to continue these observations
through long periods of time in order that everything possible to learn
may be discovered.

Our own moon is one of the enigmas of the mathematical astronomer.
Observations show that she is deviating from her predicted place, and
that this deviation continues to increase. True, it is not very great
when measured by an ordinary standard. The time at which the moon's
shadow passed a given point near Norfolk during the total eclipse of
May 29, 1900, was only about seven seconds different from the time
given in the Astronomical Ephemeris. The path of the shadow along the
earth was not out of place by more than one or two miles But, small
though these deviations are, they show that something is wrong, and no
one has as yet found out what it is. Worse yet, the deviation is
increasing rapidly. The observers of the total eclipse in August, 1905,
were surprised to find that it began twenty seconds before the
predicted time. The mathematical problems involved in correcting this
error are of such complexity that it is only now and then that a
mathematician turns up anywhere in the world who is both able and bold
enough to attack them.

There now seems little doubt that Jupiter is a miniature sun, only not
hot enough at its surface to shine by its own light The point in which
it most resembles the sun is that its equatorial regions rotate in less
time than do the regions near the poles. This shows that what we see is
not a solid body. But none of the careful observers have yet succeeded
in determining the law of this difference of rotation.

Twelve years ago a suspicion which had long been entertained that the
earth's axis of rotation varied a little from time to time was verified
by Chandler. The result of this is a slight change in the latitude of
all places on the earth's surface, which admits of being determined by
precise observations. The National Geodetic Association has established
four observatories on the same parallel of latitude--one at
Gaithersburg, Maryland, another on the Pacific coast, a third in Japan,
and a fourth in Italy--to study these variations by continuous
observations from night to night. This work is now going forward on a
well-devised plan.

A fact which will appeal to our readers on this side of the Atlantic is
the success of American astronomers. Sixty years ago it could not be
said that there was a well-known observatory on the American continent.
The cultivation of astronomy was confined to a professor here and
there, who seldom had anything better than a little telescope with
which he showed the heavenly bodies to his students. But during the
past thirty years all this has been changed. The total quantity of
published research is still less among us than on the continent of
Europe, but the number of men who have reached the highest success
among us may be judged by one fact. The Royal Astronomical Society of
England awards an annual medal to the English or foreign astronomer
deemed most worthy of it. The number of these medals awarded to
Americans within twenty-five years is about equal to the number awarded
to the astronomers of all other nations foreign to the English. That
this preponderance is not growing less is shown by the award of medals
to Americans in three consecutive years--1904, 1905, and 1906. The
recipients were Hale, Boss, and Campbell. Of the fifty foreign
associates chosen by this society for their eminence in astronomical
research, no less than eighteen--more than one-third--are Americans.




VII

LIFE IN THE UNIVERSE


So far as we can judge from what we see on our globe, the production of
life is one of the greatest and most incessant purposes of nature. Life
is absent only in regions of perpetual frost, where it never has an
opportunity to begin; in places where the temperature is near the
boiling-point, which is found to be destructive to it; and beneath the
earth's surface, where none of the changes essential to it can come
about. Within the limits imposed by these prohibitory conditions--that
is to say, within the range of temperature at which water retains its
liquid state, and in regions where the sun's rays can penetrate and
where wind can blow and water exist in a liquid form--life is the
universal rule. How prodigal nature seems to be in its production is
too trite a fact to be dwelt upon. We have all read of the millions of
germs which are destroyed for every one that comes to maturity. Even
the higher forms of life are found almost everywhere. Only small
islands have ever been discovered which were uninhabited, and animals
of a higher grade are as widely diffused as man.

If it would be going too far to claim that all conditions may have
forms of life appropriate to them, it would be going as much too far in
the other direction to claim that life can exist only with the precise
surroundings which nurture it on this planet. It is very remarkable in
this connection that while in one direction we see life coming to an
end, in the other direction we see it flourishing more and more up to
the limit. These two directions are those of heat and cold. We cannot
suppose that life would develop in any important degree in a region of
perpetual frost, such as the polar regions of our globe. But we do not
find any end to it as the climate becomes warmer. On the contrary,
every one knows that the tropics are the most fertile regions of the
globe in its production. The luxuriance of the vegetation and the
number of the animals continually increase the more tropical the
climate becomes. Where the limit may be set no one can say. But it
would doubtless be far above the present temperature of the equatorial
regions.

It has often been said that this does not apply to the human race, that
men lack vigor in the tropics. But human vigor depends on so many
conditions, hereditary and otherwise, that we cannot regard the
inferior development of humanity in the tropics as due solely to
temperature. Physically considered, no men attain a better development
than many tribes who inhabit the warmer regions of the globe. The
inferiority of the inhabitants of these regions in intellectual power
is more likely the result of race heredity than of temperature.

We all know that this earth 

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