Through Magic Glasses and Other Lectures
by Arabella B. Buckley · English · Gutenberg #37589 · 363,794 chars · public domain (US)
The Project Gutenberg eBook of Through Magic Glasses and Other Lectures
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.
Title: Through Magic Glasses and Other Lectures
Author: Arabella B. Buckley
Release date: October 1, 2011 [eBook #37589]
Language: English
Other information and formats: www.gutenberg.org/ebooks/37589
Credits: E-text prepared by Chris Curnow, Robin Shaw, and the Online Distributed Proofreading Team (http://www.pgdp.net) from page images generously made available by Internet Archive (http://www.archive.org)
*** START OF THE PROJECT GUTENBERG EBOOK THROUGH MAGIC GLASSES AND OTHER LECTURES ***
E-text prepared by Chris Curnow, Robin Shaw, and the Online Distributed
Proofreading Team (http://www.pgdp.net) from page images generously made
available by Internet Archive (http://www.archive.org)
Note: Project Gutenberg also has an HTML version of this
file which includes the original illustrations.
See 37589-h.htm or 37589-h.zip:
(https://www.gutenberg.org/cache/epub/37589/pg37589-images.html)
or
(https://www.gutenberg.org/files/37589/37589-h.zip)
Images of the original pages are available through
Internet Archive. See
http://www.archive.org/details/throughmagicglas00buck
[Illustration: For Description see Page 152 Frontispiece
THE GREAT NEBULA OF ORION
From a photograph taken on February 4th, 1889 by Mr Isaac Roberts.]
THROUGH MAGIC GLASSES AND OTHER LECTURES
A Sequel to The Fairyland of Science
by
ARABELLA B. BUCKLEY
(Mrs. Fisher)
Author of Life and Her Children, Winners in Life's Race,
A Short History of Natural Science, Etc.
With Numerous Illustrations
New York
D. Appleton and Company
1890
Authorized Edition.
PREFACE.
The present volume is chiefly intended for those of my young friends who
have read, and been interested in, the _Fairyland of Science_. It
travels over a wide field, pointing out a few of the marvellous facts
which can be studied and enjoyed by the help of optical instruments. It
will be seen at a glance that any one of the subjects dealt with might
be made the study of a lifetime, and that the little information given
in each lecture is only enough to make the reader long for more.
In these days, when moderate-priced instruments and good books and
lectures are so easily accessible, I hope some eager minds may be thus
led to take up one of the branches of science opened out to us by magic
glasses; while those who go no further will at least understand
something of the hitherto unseen world which is now being studied by
their help.
The two last lectures wander away from this path, and yet form a natural
conclusion to the Magician's lectures to his young Devonshire lads. They
have been published before, one in the _Youth's Companion_ of Boston,
U.S., and the other in _Atalanta_, in which the essay on Fungi also
appeared in a shorter form. All three lectures have, however, been
revised and fully illustrated, and I trust that the volume, as a whole,
may prove a pleasant Christmas companion.
For the magnificent photograph of Orion's nebula, forming the
Frontispiece, I am indebted to the courtesy of Mr. Isaac Roberts,
F.R.A.S., who most kindly lent me the plate for reproduction; and I have
had the great good fortune to obtain permission from MM. Henri of the
Paris Observatory to copy the illustration of the Lunar Apennines from a
most beautiful and perfect photograph of part of the moon, taken by them
only last March. My cordial thanks are also due to Mr. A. Cottam,
F.R.A.S., for preparing the plate of coloured double stars, and to my
friend Mr. Knobel, Hon. Sec. of the R.A.S., for much valuable
assistance; to Mr. James Geikie for the loan of some illustrations from
his _Geology_; and to Messrs. Longman for permission to copy Herschel's
fine drawing of Copernicus.
With the exception of these illustrations and a few others, three of
which were kindly given me by Messrs. Macmillan, all the woodcuts have
been drawn and executed under the superintendence of Mr. Carreras, jun.,
who has made my task easier by the skill and patience he has exercised
under the difficulties incidental to receiving instructions from a
distance.
ARABELLA B. BUCKLEY.
UPCOTT AVENEL, _Oct. 1890_.
TABLE OF CONTENTS
CHAPTER I PAGE
THE MAGICIAN'S CHAMBER BY MOONLIGHT 1
CHAPTER II
MAGIC GLASSES AND HOW TO USE THEM 27
CHAPTER III
FAIRY RINGS AND HOW THEY ARE MADE 55
CHAPTER IV
THE LIFE-HISTORY OF LICHENS AND MOSSES 75
CHAPTER V
THE HISTORY OF A LAVA STREAM 96
CHAPTER VI
AN HOUR WITH THE SUN 117
CHAPTER VII
AN EVENING AMONG THE STARS 145
CHAPTER VIII
LITTLE BEINGS FROM A MINIATURE OCEAN 172
CHAPTER IX
THE DARTMOOR PONIES 195
CHAPTER X
THE MAGICIAN'S DREAM OF ANCIENT DAYS 209
LIST OF ILLUSTRATIONS
PLATES
PHOTOGRAPH OF THE NEBULA OF ORION _Frontispiece_
TABLE OF COLOURED SPECTRA Plate I. _facing p._ 127
COLOURED DOUBLE STARS Plate II. _facing p._ 167
WOODCUTS IN THE TEXT PAGE
PARTIAL ECLIPSE OF THE MOON _Initial letter_ 1
A BOY ILLUSTRATING THE PHASES OF THE MOON 6
COURSE OF THE MOON IN THE HEAVENS 8
CHART OF THE MOON 10
FACE OF THE FULL MOON 11
TYCHO AND HIS SURROUNDINGS (from a photograph by De la Rue) 13
PLAN OF THE PEAK OF TENERIFFE 15
THE CRATER COPERNICUS 17
THE LUNAR APPENNINES (from a photograph by M.M. Henri) 19
THE CRATER PLATO SEEN SOON AFTER SUNRISE 20
DIAGRAM OF TOTAL ECLIPSE OF THE MOON 23
BOY AND MICROSCOPE _Initial letter_ 27
EYE-BALL SEEN FROM THE FRONT 30
SECTION OF AN EYE LOOKING AT A PENCIL 31
IMAGE OF A CANDLE-FLAME THROWN ON PAPER BY A LENS 33
ARROW MAGNIFIED BY A CONVEX LENS 35
STUDENT'S MICROSCOPE 36
SKELETON OF A MICROSCOPE 37
FOSSIL DIATOMS SEEN UNDER THE MICROSCOPE 39
AN ASTRONOMICAL TELESCOPE 41
TWO SKELETONS OF TELESCOPES 44
THE PHOTOGRAPHIC CAMERA 47
KIRCHHOFF'S SPECTROSCOPE 51
PASSAGE OF RAYS THROUGH THE SPECTROSCOPE 52
A GROUP OF FAIRY-RING MUSHROOMS _Initial letter_ 55
THREE FORMS OF VEGETABLE MOULD MAGNIFIED 61
_MUCOR MUCEDO_ GREATLY MAGNIFIED 63
YEAST CELLS GROWING UNDER THE MICROSCOPE 65
EARLY STAGES OF THE MUSHROOM 67
LATER STAGES OF THE MUSHROOM 68
MICROSCOPIC STRUCTURE OF MUSHROOM GILLS 69
A GROUP OF CUP LICHENS _Initial letter_ 75
EXAMPLES OF LICHENS FROM LIFE 77
SINGE-CELLED PLANTS GROWING 78
SECTIONS OF LICHENS 81
FRUCTIFICATION OF A LICHEN 83
A STEM OF FEATHERY MOSS FROM LIFE 85
MOSS-LEAF MAGNIFIED 87
_POLYTRICHUM COMMUNE_, A LARGE HAIR-MOSS 88
FRUCTIFICATION OF A MOSS 89
SPHAGNUM MOSS FROM A DEVONSHIRE BOG 93
SURFACE OF A LAVA-FLOW _Initial letter_ 96
VESUVIUS AS SEEN IN ERUPTION 97
TOP OF VESUVIUS IN 1864 100
DIAGRAMMATIC SECTION OF AN ACTIVE VOLCANO 105
SECTION OF A LAVA-FLOW 108
VOLCANIC GLASS WITH CRYSTALLITES AND MICROLITHS 109
VOLCANIC GLASS WITH WELL-DEVELOPED MICROLITHS 110
A PIECE OF DARTMOOR GRANITE 112
VOLCANIC GLASS SHOWING LARGE INCLUDED CRYSTALS 115
A TOTAL ECLIPSE OF THE SUN _Initial letter_ 117
FACE OF THE SUN PROJECTED ON A PIECE OF CARDBOARD 120
PHOTOGRAPH OF THE SUN'S FACE, taken by Mr. Selwyn
(Secchi, _Le Soleil_) 122
TOTAL ECLIPSE OF THE SUN, SHOWING CORONA AND PROMINENCES
(Guillemin, _Le Ciel_) 124
KIRCHHOFF'S EXPERIMENT ON THE DARK SODIUM LINE 128
THE SPECTROSCOPE ATTACHED TO THE TELESCOPE FOR SOLAR WORK 132
SUN-SPECTRUM AND PROMINENCE SPECTRUM COMPARED 134
RED PROMINENCES, as drawn by Mr. Lockyer 1869 136
A QUIET SUN-SPOT 140
A TUMULTUOUS SUN-SPOT 141
A STAR-CLUSTER _Initial letter_ 145
SOME CONSTELLATIONS SEEN ON LOOKING SOUTH IN MARCH FROM
SIX TO NINE O'CLOCK 148
THE CHIEF STARS OF ORION, WITH ALDEBARAN 149
THE TRAPEZIUM [Greek: th] ORIONIS 150
SPECTRUM OF ORION'S NEBULA AND SUN-SPECTRUM COMPARED 151
SOME CONSTELLATIONS SEEN ON LOOKING NORTH IN MARCH FROM
SIX TO NINE O'CLOCK 156
THE GREAT BEAR, SHOWING POSITION OF THE BINARY STAR 157
DRIFTING OF THE SEVEN STARS OF CHARLES'S WAIN 159
CASSIOPEIA AND THE HEAVENLY BODIES NEAR 162
[Greek: e] LYRAE, A DOUBLE-BINARY STAR 166
A SEASIDE POOL _Initial letter_ 172
A GROUP OF SEAWEEDS (natural size) 175
_ULVA LACTUCA_, a piece greatly magnified 176
SEAWEEDS, magnified to show fruits 177
A CORALLINE AND SERTULARIAN COMPARED 179
_SERTULARIA TENELLA_ HANGING IN WATER 180
_THURICOLLA FOLLICULATA_ AND _CHILOMONAS AMYGDALUM_ 182
A GROUP OF LIVING DIATOMS 184
A DIATOM GROWING 185
_CYDIPPE PILEUS_, ANIMAL AND STRUCTURE 187
THE SEA-MAT, _FLUSTRA FOLIACEA_ 191
DIAGRAM OF THE FLUSTRA ANIMAL 192
DARTMOOR PONIES _Initial letter_ 195
_EQUUS HEMIONUS_, THE HORSE-ASS OF TARTARY AND TIBET 201
PRZEVALSKY'S WILD HORSE 202
SKELETON OF AN ANIMAL OF THE HORSE-TRIBE 206
PALAEOLITHIC MAN CHIPPING FLINT TOOLS _Initial letter_ 209
SCENE IN PALAEOLITHIC TIMES 212
PALAEOLITHIC RELICS--NEEDLE, TOOTH, IMPLEMENT 213
MAMMOTH ENGRAVED ON IVORY 216
NEOLITHIC IMPLEMENTS--HATCHET, CELT, SPINDLE WHORL 219
A BURIAL IN NEOLITHIC TIMES 221
BRITISH RELICS--COIN, BRONZE CELT, AND BRACELET 223
BRITONS TAKING REFUGE IN THE CAVE 224
THROUGH MAGIC GLASSES
CHAPTER I
THE MAGICIAN'S CHAMBER BY MOONLIGHT
[Illustration]
The full moon was shining in all its splendour one lovely August night,
as the magician sat in his turret chamber bathed in her pure white
beams, which streamed upon him through the open shutter in the wooden
dome above. It is true a faint gleam of warmer light shone from below
through the open door, for this room was but an offshoot at the top of
the building, and on looking down the turret stairs a lecture-room might
be seen below where a bright light was burning. Very little, however, of
this warm glow reached the magician, and the implements of his art
around him looked like weird gaunt skeletons as they cast their long
shadows across the floor in the moonlight.
The small observatory, for such it was, was a circular building with
four windows in the walls, and roofed with a wooden dome, so made that
it could be shifted round and round by pulling certain cords. One
section of this dome was a shutter, which now stood open, and the strip,
thus laid bare to the night, was so turned as to face that part of the
sky along which the moon was moving. In the centre of the room, with its
long tube directed towards the opening, stood the largest magic glass,
the TELESCOPE, and in the dead stillness of the night, could be heard
distinctly the tick-tick of the clockwork, which kept the instrument
pointing to the face of the moon, while the room, and all in it, was
being carried slowly and steadily onwards by the earth's rotation on its
axis. It was only a moderate-sized instrument, about six feet long,
mounted on a solid iron pillar firmly fixed to the floor and fitted with
the clockwork, the sound of which we have mentioned; yet it looked like
a giant as the pale moonlight threw its huge shadow on the wall behind
and the roof above.
Far away from this instrument in one of the windows, all of which were
now closed with shutters, another instrument was dimly visible. This was
a round iron table, with clawed feet, and upon it, fastened by screws,
were three tubes, so arranged that they all pointed towards the centre
of the table, where six glass prisms were arranged in a semicircle, each
one fixed on a small brass tripod. A strange uncanny-looking instrument
this, especially as the prisms caught the edge of the glow streaming up
the turret stair, and shot forth faint beams of coloured light on the
table below them. Yet the magician's pupils thought it still more
uncanny and mysterious when their master used it to read the alphabet of
light, and to discover by vivid lines even the faintest trace of a metal
otherwise invisible to mortal eye.
For this instrument was the SPECTROSCOPE, by which he could break up
rays of light and make them tell him from what substances they came.
Lying around it were other curious prisms mounted in metal rims and
fitted with tubes and many strange devices, not to be understood by the
uninitiated, but magical in their effect when fixed on to the telescope
and used to break up the light of distant stars and nebulae.
Compared with these mysterious glasses the PHOTOGRAPHIC CAMERA, standing
in the background, with its tall black covering cloth, like a hooded
monk, looked comparatively natural and familiar, yet it, too, had
puzzling plates and apparatus on the table near it, which could be
fitted on to the telescope, so that by their means pictures might be
taken even in the dark night, and stars, invisible with the strongest
lens, might be forced to write their own story, and leave their image on
the plate for after study.
All these instruments told of the magician's power in unveiling the
secrets of distant space and exploring realms unknown, but in another
window, now almost hidden in the shadow, stood a fourth and
highly-prized helpmate, which belonged in one sense more to our earth,
since everything examined by it had to be brought near, and lie close
under its magnifying-glass. Yet the MICROSCOPE too could carry its
master into an unseen world, hidden to mortal eye by minuteness instead
of by distance. If in the stillness of night the telescope was his most
cherished servant and familiar friend, the microscope by day opened out
to him the fairyland of nature.
As he sat on his high pedestal stool on this summer night with the
moonlight full upon him, his whole attention was centred on the
telescope, and his mind was far away from that turret-room, wandering
into the distant space brought so near to him; for he was waiting to
watch an event which brought some new interest every time it took
place--a total eclipse of the moon. To-night he looked forward to it
eagerly, for it happened that, just as the moon would pass into the
shadow of our earth, it would also cross directly in front of a star,
causing what is known as an "occultation" of the star, which would
disappear suddenly behind the rim of the dark moon, and after a short
time flash out on the other side as the satellite went on its way.
How he wished as he sat there that he could have shown this sight to all
the eager lads whom he was teaching to handle and love his magic
glasses. For this magician was not only a student himself, he was a rich
man and the Founder and Principal of a large public school for boys of
the artisan class. He had erected a well-planned and handsome building
in the midst of the open country, and received there, on terms within
the means of their parents, working-lads from all parts of England, who,
besides the usual book-learning, received a good technical education in
all its branches. And, while he left to other masters the regular
school lessons, he kept for himself the intense pleasure of opening the
minds of these lads to the wonders of God's universe around them.
You had only to pass down the turret stairs, into the large science
class-room below, to see at once that a loving hand and heart had
furnished it. Not only was there every implement necessary for
scientific work, but numerous rough diagrams covering the walls showed
that labour as well as money had been spent in decorating them. It was a
large oblong room, with four windows to the north, and four to the
south, in each of which stood a microscope with all the tubes, needles,
forceps, knives, etc., necessary for dissecting and preparing objects;
and between the windows were open shelves, on which were ranged
chemicals of various kinds, besides many strange-looking objects in
bottles, which would have amused a trained naturalist, for the lads
collected and preserved whatever took their fancy.
On some of the tables were photographic plates laid ready for printing
off; on others might be seen drawings of the spectrum, made from the
small spectroscope fixed at one end of the room; on others lay small
direct spectroscopes which the lads could use for themselves. But
nowhere was a telescope to be seen. This was not because there were
none, for each table had its small hand-telescope, cheap but good. The
truth is that each of these instruments had been spirited away into the
dormitories that night, and many heads were lying awake on their
pillows, listening for the strike of the clock to spring out and see
the eclipse begin.
[Illustration: Fig. 1.
A boy illustrating the phases of the moon.]
A mere glance round the room showed that the moon had been much studied
lately. On the black-board was drawn a rough diagram, showing how a boy
can illustrate for himself the moon's journey round the earth, by taking
a ball and holding it a little above his head at arm's length, while he
turns slowly round on his heel in a darkened room before a lighted lamp,
or better still before the lens of a magic lantern (Fig. 1). The lamp or
lens then represents the sun, the ball is the moon, the boy's head is
the earth. Beginning with the ball between him and the source of light,
but either a little above, or a little below the direct line between his
eye and it, he will see only the dark side of the ball, and the moon
will be on the point of being "new." Then as he turns slowly, a thin
crescent of light will creep over the side nearest the sun, and by
degrees encroach more and more, so that when he has turned through one
quarter of the round half the disc will be light. When he has turned
another quarter, and has his back to the sun, a full moon will face him.
Then as he turns on through the third quarter a crescent of darkness
creeps slowly over the side away from the sun, and gradually the bright
disc is eaten away by shadow till at the end of the third quarter half
the disc again only is light; then, when he has turned through another
quarter and completed the circle, he faces the light again and has a
dark moon before him. But he must take care to keep the moon a little
above or a little below his eye at new and full moon. If he brings it
exactly on a line with himself and the light at new moon, he will shut
off the light from himself and see the dark body of the ball against the
light, causing an _eclipse_ of the sun; while if he does the same at
full moon his head will cast a shadow on the ball causing an _eclipse_
of the moon.
There were other diagrams showing how and why such eclipses do really
happen at different times in the moon's path round the earth; but
perhaps the most interesting of all was one he had made to explain what
so few people understand, namely, that though the moon describes a
complete circle round our earth every month, yet she does not describe a
circle in space, but a wavy line inwards and outwards across the
earth's path round the sun. This is because the earth is moving on all
the while, carrying the moon with it, and it is only by seeing it drawn
before our eyes that we can realise how it happens.
Thus suppose, in order to make the dates as simple as possible, that
there is a new moon on the 1st of some month. Then by the 9th (or
roughly speaking in 7-3/4 days) the moon will have described a quarter
of a circle round the earth as shown by the dotted line (Fig. 2), which
marks her position night after night with regard to us. Yet because she
is carried onwards all the while by the earth, she will really have
passed along the interrupted line - - - - between us and the sun. During
the next week her quarter of a circle will carry her round behind the
earth, so that we see her on the 17th as a full moon, yet her actual
movement has been onwards along the interrupted line on the farther side
of the earth. During the third week she creeps round another quarter of
a circle so as to be in advance of the earth on its yearly journey round
the sun, and reaches the end of her third quarter on the 24th. In her
last quarter she gradually passes again between the earth and the sun;
and though, as regards the earth, she appears to be going back round to
the same place where she was at the beginning of the month, and on the
31st is again a dark new moon, yet she has travelled onwards exactly as
much as we have, and therefore has really not described a circle in the
_heavens_ but a wavy line.
[Illustration: Fig. 2.
Diagram showing the moon's course during one month. The moon and the
earth are both moving onwards in the direction of the arrows. The earth
moves along the dark line, the moon along the interrupted line - - - -.
The dotted curved line .... shows the circle gradually described by the
moon round the earth as they move onwards.]
Near to this last diagram hung another, well loved by the lads, for it
was a large map of the _face_ of the moon, that is of the side which is
_always_ turned towards us, because the moon turns once on her axis
during the month that she is travelling round the earth. On this map
were marked all the different craters, mountains, plains and shining
streaks which appear on the moon's face; while round the chart were
pictures of some of these at sunrise and sunset on the moon, or during
the long day of nearly a fortnight which each part of the face enjoys in
its turn.
[Illustration: Fig. 3.
Chart of the moon.
Craters--
1 Tycho. 4 Aristarchus. 7 Plato. 10 Petavius.
2 Copernicus. 5 Eratosthenes. 8 Eudoxus. 11 Ptolemy.
3 Kepler. 6 Archimedes. 9 Aristotle.
Grey plains formerly believed to be seas--
A Mare Crisium. O Mare Imbrium.
C ---- Frigoris. Q Oceanus Procellarum.
G ---- Tranquillitatis. X Mare Foecunditatis.
H ---- Serenitatis. T ---- Humorum.]
By studying this map, and the pictures, they were able, even in their
small telescopes, to recognise Tycho and Copernicus, and the mountains
of the moon, after they had once grown accustomed to the strange
changes in their appearance which take place as daylight or darkness
creeps over them. They could not however pick out more than some of the
chief points. Only the magician himself knew every crater and ridge
under all its varying lights, and now, as he waited for the eclipse to
begin, he turned to a lad who stood behind him, almost hidden in the
dark shadow--the one fortunate boy who had earned the right to share
this night's work.
[Illustration: Fig. 3_a_.
The full moon. (From Ball's _Starland_.)]
"We have still half an hour, Alwyn," said he, "before the eclipse will
begin, and I can show you the moon's face well to-night. Take my place
here and look at her while I point out the chief features. See first,
there are the grey plains (A, C, G, etc.) lying chiefly in the lower
half of the moon. You can often see these on a clear night with the
naked eye, but you must remember that then they appear more in the upper
part, because in the telescope we see the moon's face inverted or upside
down.
"These plains were once thought to be oceans, but are now proved to be
dry flat regions situated at different levels on the moon, and much like
what deserts and prairies would appear on our earth if seen from the
same distance. Looking through the telescope, is it not difficult to
imagine how people could ever have pictured them as a man's face? But
not so difficult to understand how some ancient nations thought the moon
was a kind of mirror, in which our earth was reflected as in a
looking-glass, with its seas and rivers, mountains and valleys; for it
does look something like a distant earth, and as the light upon it is
really reflected from the sun it was very natural to compare it to a
looking-glass.
"Next cast your eye over the hundreds of craters, some large, others
quite small, which cover the moon's face with pitted marks, like a man
with small-pox; while a few of the larger rings look like holes made in
a window-pane, where a stone has passed through, for brilliant shining
streaks radiate from them on all sides like the rays of a star, covering
a large part of the moon. Brightest of all these starred craters is
Tycho, which you will easily find near the top of the moon (I, Fig. 3),
for you have often seen it in the small telescope. How grand it looks
to-night in the full moon (Fig. 3_a_)! It is true you see all the
craters better when the moon is in her quarters, because the light falls
sideways upon them and the shadows are more sharply defined; yet even at
the full the bright ray of light on Tycho's rim marks out the huge
cavity, and you can even see faintly the magnificent terraces which run
round the cup within, one below the other."
[Illustration: Fig. 4.
Tycho and his surroundings. (From a photograph of the moon taken by Mr.
De la Rue, 1863.)]
"This cavity measures fifty-four miles across, so that if it could be
moved down to our earth it would cover by far the largest part of
Devonshire, or that portion from Bideford on the north, to the sea on
the south, and from the borders of Cornwall on the east, to Exeter on
the west, and it is 17,000 feet or nearly three miles in depth. Even in
the brilliant light of the full moon this enormous cup is dark compared
to the bright rim, but it is much better seen in about the middle of the
second quarter, when the rising sun begins to light up one side while
the other is in black night. The drawing on the wall (Fig. 4), which is
taken from an actual photograph of the moon's face, shows Tycho at this
time surrounded by the numerous other craters which cover this part of
the moon. You may recognise him by the gleaming peak in the centre of
the cup, and by his bright rim which is so much more perfect than those
of his companions. The gleaming peak is the top of a steep cone or hill
rising up 6000 feet, or more than a mile from the base of the crater, so
that even the summit is about two miles below the rim.
"There is one very interesting point in Tycho, however, which is seen at
its very best at full moon. Look outside the bright rim and you will see
that from the shadow which surrounds it there spring on all sides those
strange brilliant streaks (see Fig. 3_a_) which I spoke of just now.
There are others quite as bright, or even brighter, round other craters,
Copernicus (Fig. 6), Kepler, and Aristarchus, lower down on the
right-hand side of the moon; but these of Tycho are far the most widely
spread, covering almost all the top of the face.
"What are these streaks? We do not know. During the second quarter of
the moon, when the sun is rising slowly upon Tycho, lighting up his
peak and showing the crater beautifully divided into a bright cup in the
curve to the right, while a dense shadow lies in the left hollow, these
streaks are only faint, and among the many craters around (see Fig. 4)
you might even have some difficulty at first in finding the well-known
giant. But as the sun rises higher and higher they begin to appear, and
go on increasing in brightness till they shine with that wonderfully
silvery light you see now in the full moon."
[Illustration: Fig. 5.
Plan of the Peak of Teneriffe, showing how it resembles a lunar crater.
(A. Geikie.)]
"Here is a problem for you young astronomers to solve, as we learn more
and more how to use the telescope with all its new appliances."
The crater itself is not so difficult to explain, for we have many like
it on our earth, only not nearly so large. In fact, we might almost say
that our earthly volcanoes differ from those in the moon only by their
smaller size and by forming _mountains_ with the crater or cup on the
top; while the lunar craters lie flat on the surface of the moon, the
hollow of the cup forming a depression below it. The peak of Teneriffe
(Fig. 5), which is a dormant volcano, is a good copy in miniature on our
earth of many craters on the moon. The large plain surrounded by a high
rocky wall, broken in places by lava streams, the smaller craters
nestling in the cup, and the high peak or central crater rising up far
above the others, are so like what we see on the moon that we cannot
doubt that the same causes have been at work in both cases, even though
the space enclosed in the rocky wall of Teneriffe measures only eight
miles across, while that of Tycho measures fifty-four.
"But of the streaks we have no satisfactory explanation. They pass alike
over plain and valley and mountain, cutting even across other craters
without swerving from their course. The astronomer Nasmyth thought they
were the remains of cracks made when the volcanoes were active, and
filled with molten lava from below, as water oozes up through ice-cracks
on a pond. But this explanation is not quite satisfactory, for the lava,
forcing its way through, would cool in ridges which ought to cast a
shadow in sunlight. These streaks, however, not only cast no shadow, as
you can see at the full moon but when the sun shines sideways upon them
in the new or waning moon they disappear as we have seen altogether.
Thus the streaks, so brilliant at full moon in Tycho, Copernicus,
Kepler, and Aristarchus, remain a puzzle to astronomers still."
[Illustration: Fig. 6.
The crater Copernicus. (As given in Herschel's _Astronomy_, from a
drawing taken in a reflecting telescope of 20 feet focal length.)]
"We cannot examine these three last-named craters well to-night with the
full sun upon them; but mark their positions well, for Copernicus, at
least, you must examine on the first opportunity, when the sun is
rising upon it in the moon's second quarter. It is larger even than
Tycho, measuring fifty-six miles across, and has a hill in the centre
with many peaks; while outside, great spurs or ridges stretch in all
directions sometimes for more than a hundred miles, and between these
are scattered innumerable minute craters. But the most striking feature
in it is the ring, which is composed inside the crater of magnificent
terraces divided by deep ravines. These terraces are in some ways very
like those of the great crater of Teneriffe, and astronomers can best
account for them by supposing that this immense crater was once filled
with a lake of molten lava rising, cooling at the edges, and then
falling again, leaving the solid ridge behind. The streaks are also
beautifully shown in Copernicus (see Fig. 6), but, as in Tycho, they
fade away as the sun sets on the crater, and only reappear gradually as
midday approaches.
"And now, looking a little to the left of Copernicus, you will see that
grand range of mountains, the Lunar Apennines (Fig. 7), which stretches
400 miles across the face of the moon. Other mountain ranges we could
find, but none so like mountains on our own globe as these, with their
gentle sunny slope down to a plain on the left, and steep perpendicular
cliffs on the right. The highest peak in this range, called Huyghens,
rises to the height of 21,000 feet, higher than Chimborazo in the Andes.
Other mountains on the moon, such as those called the Caucasus, south of
the Apennines, are composed of disconnected peaks, while others again
stand as solitary pyramids upon the plains."
[Illustration: Fig. 7.
The Lunar Apennines.
(Copied by kind permission of MM. Henri from part of a magnificent
photograph taken by them, March 29, 1890, at the Paris Observatory.)]
"But we must hasten on, for I want you to observe those huge walled
crater-plains which have no hill in the middle, but smooth steel-grey
centres shining like mirrors in the moonlight. One of these, called
Archimedes, you will find just below the Lunar Apennines (Figs. 3 and
7), and another called Plato, which is sixty miles broad, is still lower
down the moon's face (Figs. 3 and 8). The centres of these broad
circles are curiously smooth and shining like quicksilver, with minute
dots here and there which are miniature craters, while the walls are
rugged and crowned with turret-shaped peaks."
[Illustration: Fig. 8.
The crater Plato as seen soon after sunrise. (After Neison.)]
"It is easy to picture to oneself how these may once have been vast seas
of lava, not surging as in Copernicus, and heaving up as it cooled into
one great central cone, but seething as molten lead does in a crucible,
little bubbles bursting here and there into minute craters; and this is
the explanation given of them by astronomers.
"And now that you have seen the curious rugged face of the moon and its
craters and mountains, you will want to know how all this has come
about. We can only form theories on the point, except that everything
shows that heat and volcanoes have in some way done the work, though no
one has ever yet clearly proved that volcanic eruptions have taken place
in our time. We must look back to ages long gone by for those mighty
volcanic eruptions which hurled out stones and ashes from the great
crater of Tycho, and formed the vast seas of lava in Copernicus and
Plato.
"And when these were over, and the globe was cooling down rapidly, so
that mountain ranges were formed by the wrinkling and rending of the
surface, was there then any life on the moon? Who can tell? Our magic
glasses can reveal what now is, so far as distance will allow; but what
has been, except where the rugged traces remain, we shall probably never
know. What we now see is a dead worn-out planet, on which we cannot
certainly trace any activity except that of heat in the past. That there
is no life there now, at any rate of the kind on our own earth, we are
almost certain; first, because we can nowhere find traces of water,
clouds, nor even mist, and without moisture no life like ours is
possible; and secondly, because even if there is, as perhaps there may
be, a thin ocean of gas round the moon there is certainly no atmosphere
such as surrounds our globe.
"One fact which proves this is, that there are no half-shadows on the
moon. If you look some night at the mountains and craters during her
first and second quarters, you will be startled to see what heavy
shadows they cast, not with faint edges dying away into light, but sharp
and hard (see Figs. 6-8), so that you pass, as it were by one step, from
shadow to sunshine. This in itself is enough to show that there is no
air to scatter the sunlight and spread it into the edges of the shade as
happens on our earth; but there are other and better proofs. One of
these is, that during an eclipse of the sun there is no reflection of
his light round the dark moon as there would be if the moon had an
atmosphere; another is that the spectroscope, that wonderful instrument
which shows us invisible gases, gives no hint of air around the moon;
and another is the sudden disappearance or _occultation_ of a star
behind the moon, such as I hope to see in a few minutes.
"See here! take the small hand telescope and turn it on to the moon's
face while I take my place at the large one, and I will tell you what to
look for. You know that at sunset we see the sun for some time after it
has dipped below the horizon, because the rays of light which come from
it are bent in our atmosphere and brought to our eyes, forming in them
the image of the sun which is already gone. Now in a short time the moon
which we are watching will be darkened by our earth coming between it
and the sun, and while it is quite dark it will pass over a little
bright star. In fact to us the star will appear to set behind the dark
moon as the sun sets below the horizon, and if the moon had an
atmosphere like ours, the rays from the star would be bent in it and
reach our eyes after the star was gone, so that it would only disappear
gradually. Astronomers have always observed, however, that the star is
lost to sight quite suddenly, showing that there is no ocean of air
round the moon to bend the light-rays."
[Illustration: Fig. 9.
Diagram of total eclipse of the moon.
S, Sun. E, Earth. M, Moon passing into the earth's shadow and passing
out at M'.
R, R', Lines meeting at a point U, U' behind the earth and enclosing a
space within which all the direct rays of the sun are intercepted by the
earth, causing a black darkness or _umbra_.
R, P and R', P', Lines marking a space within which, behind the earth,
part of the sun's rays are cut off, causing a half-shadow or _penumbra_,
P, P'.
_a_, _a_, Points where a few of the sun's rays are bent or refracted in
the earth's atmosphere, so that they pass along the path marked by the
dotted lines and shed a lurid light on the sun's face.]
Here the magician paused, for a slight dimness on the lower right-hand
side of the moon warned him that she was entering into the _penumbra_ or
half-shadow (see Fig. 9) caused by the earth cutting off part of the
sun's rays; and soon a deep black shadow creeping over Aristarchus and
Plato showed that she was passing into that darker space or _umbra_
where the body of the earth is completely between her and the sun and
cuts off all his rays. All, did I say? No! not all. For now was seen a
beautiful sight, which would prove to any one who saw our earth from a
great distance that it has a deep ocean of air round it.
It was a clear night, with a cloudless sky, and as the deep shadow crept
slowly over the moon's face, covering the Lunar Apennines and
Copernicus, and stealing gradually across the brilliant streaks of Tycho
till the crater itself was swallowed up in darkness, a strange lurid
light began to appear. The part of the moon which was eclipsed was not
wholly dark, but tinted with a very faint bluish-green light, which
changed almost imperceptibly, as the eclipse went on, to rose-red, and
then to a fiery copper-coloured glow as the moon crept entirely into the
shadow and became all dark. The lad watching through his small telescope
noted this weird light, and wondered, as he saw the outlines of the
Apennines and of several craters dimly visible by it, though the moon
was totally eclipsed. He noted, but was silent. He would not disturb the
Principal, for the important moment was at hand, as this dark
copper-coloured moon, now almost invisible, drew near to the star over
which it was to pass.
This little star, really a glorious sun billions of miles away behind
the moon, was perhaps the centre of another system of worlds as unknown
to us as we to them, and the fact of our tiny moon crossing between it
and our earth would matter as little as if a grain of sand was blown
across the heavens. Yet to the watchers it was a great matter--would the
star give any further clue to the question of an atmosphere round the
moon? Would its light linger even for a moment, like the light of the
setting sun? Nearer and nearer came the dark moon; the star shone
brilliantly against its darkness; one second and it was gone. The long
looked-for moment had passed, and the magician turned from his
instrument with a sigh. "I have learnt nothing new, Alwyn," said he,
"but at least it is satisfactory to have seen for ourselves the proof
that there is no perceptible atmosphere round the moon. We need wait no
longer, for before the star reappears on the other side the eclipse will
be passing away."
"But, master," burst forth the lad, now the silence was broken, "tell me
why did that strange light of many tints shine upon the dark moon?"
"Did you notice it, Alwyn?" said the Principal, with a pleased smile.
"Then our evening's work is not lost, for you have made a real
observation for yourself. That light was caused by the few rays of the
sun which grazed the edge of our earth passing through the ocean of air
round it (see Fig. 9). There they were refracted or bent, and so were
thrown within the shadow cast by our earth, and fell upon the moon. If
there were such a person as a 'man in the moon,' that lurid light would
prove to him that our earth has an atmosphere. The cause of the tints is
the same which gives us our sunset colours, because as the different
coloured waves which make white light are absorbed one by one, passing
through the denser atmosphere, the blue are cut off first, then the
green, then the yellow, till only the orange and red rays reached the
centre of the shadow, where the moon was darkest. But this is too
difficult a subject to begin at midnight."
So saying, he lighted his lamp, and covering the object-glass of his
telescope with its pasteboard cap, detached the instrument from the
clockwork, and the master and his pupil went down the turret stairs and
past through the room below. As they did so they heard in the distance a
scuffling noise like that of rats in the wall. A smile passed over the
face of the Principal, for he knew that his young pupils, who had been
making their observations in the gallery above, were hurrying back to
their beds.
CHAPTER II
MAGIC GLASSES, AND HOW TO USE THEM
[Illustration]
The sun shone brightly into the science class-room at mid-day. No gaunt
shadows nor ghostly moonlight now threw a spell on the magic chamber
above. The instruments looked bright and business-like, and the
Principal, moving amongst them, heard the subdued hum of fifty or more
voices rising from below. It was the lecture hour, and the subject for
the day was, "Magic glasses, and how to use them." As the large clock in
the hall sounded twelve, the Principal gathered up a few stray lenses
and prisms he had selected, and passed down the turret stair to his
platform. Behind him were arranged his diagrams, before him on the table
stood various instruments, and the rows of bright faces beyond looked up
with one consent as the hum quieted down and he began his lecture.
"I have often told you, boys, have I not? that I am a Magician. In my
chamber near the sky I work spells as did the magicians of old, and by
the help of my magic glasses I peer into the secrets of nature. Thus I
read the secrets of the distant stars; I catch the light of wandering
comets, and make it reveal its origin; I penetrate into the whirlpools
of the sun; I map out the craters of the moon. Nor can the tiniest being
on earth hide itself from me. Where others see only a drop of muddy
water, that water brought into my magic chamber teems with thousands of
active bodies, darting here and whirling there amid a meadow of tiny
green plants floating in the water. Nay, my inquisitive glass sees even
farther than this, for with it I can watch the eddies of water and green
atoms going on in each of these tiny beings as they feed and grow.
Again, if I want to break into the secrets of the rock at my feet, I
have only to put a thin slice of it under my microscope to trace every
crystal and grain; or, if I wish to learn still more, I subject it to
fiery heat, and through the magic prisms of my spectroscope I read the
history of the very substances of which it is composed. If I wish to
study the treasures of the wide ocean, the slime from a rock-pool teems
with fairy forms darting about in the live box imprisoned in a crystal
home. If some distant stars are invisible even in the giant glasses of
my telescope, I set another power to work, and make them print their own
image on a photographic plate and so reveal their presence.
"All these things you have seen through my magic glasses, and I
promised you that one day I would explain to you how they work and do my
bidding. But I must warn you that you must give all your attention;
there is no royal road to my magician's power. Every one can attain to
it, but only by taking trouble. You must open your eyes and ears, and
use your intelligence to test carefully what your senses show you.
"We have only to consider a little to see that we depend entirely upon
our senses for our knowledge of the outside world. All kinds of things
are going on around us, about which we know nothing, because our eyes
are not keen enough to see, and our ears not sharp enough to hear them.
Most of all we enjoy and study nature through our eyes, those windows
which let in to us the light of heaven, and with it the lovely sights
and scenes of earth; and which are no ordinary windows, but most
wonderful structures adapted for conveying images to the brain. They are
of very different power in different people, so that a long-sighted
person sees a lovely landscape where a short-sighted one sees only a
confused mist; while a short-sighted person can see minute things close
to the eye better than a long-sighted one."
[Illustration: Fig. 10.
Eye-ball seen from the front. (After Le Gros Clark.)
_w_, White of eye. _i_, Iris. _p_, Pupil.]
"Let us try to understand this before we go on to artificial glasses,
for it will help us to explain how these glasses show us many things we
could never see without them. Here are two pictures of the human eyeball
(Figs. 10 and 11), one as it appears from the front, and the other as we
should see the parts if we cut an eyeball across from the front to the
back. From these drawings we see that the eyeball is round; it only
looks oval, because it is seen through the oval slit of the eyelids.
It is really a hard, shining, white ball with a thick nerve cord
(_on_, Fig. 11) passing out at the back, and a dark glassy mound
_c_, _c_ in the centre of the white in front. In this mound we can
easily distinguish two parts--first, the coloured _iris_ or elastic
curtain (_i_, Fig. 10); and secondly, the dark spot or pupil _p_ in the
centre. The iris is the part which gives the eye its colour; it is
composed of a number of fibres, the outer ones radiating towards the
centre, the inner ones forming a ring round the pupil; and behind these
fibres is a coat of dark pigment or colouring matter, blue in some
people, grey, brown, or black in others. When the light is very strong,
and would pain the nerves inside if too much entered the pupil or window
of the eye, then the ring of the iris contracts so as partly to close
the opening. When there is very little light, and it is necessary to let
in as much as possible, the ring expands and the pupil grows large. The
best way to observe this is to look at a cat's eyes in the dusk, and
then bring her near to a bright light; for the iris of a cat's eye
contracts and expands much more than ours does."
[Illustration: Fig. 11.
Section of an eye looking at a pencil. (Adapted from Kirke.)
_c_, _c_, Cornea. _w_, White of eye. _cm_, Ciliary muscle. _a_, _a_,
Aqueous humour. _i_, _i_, Iris. _l_, _l_, Lens. _r_, _r_, Retina. _on_,
Optic nerve. 1, 2, Pencil. 1', 2', Image of pencil on the retina.]
"Now look at the second diagram (Fig. 11) and notice the chief points
necessary in seeing. First you will observe that the pupil is not a mere
hole; it is protected by a curved covering _c_. This is the cornea, a
hard, perfectly transparent membrane, looking much like a curved
watch-glass. Behind this is a small chamber filled with a watery fluid
_a_, called the aqueous humour, and near the back of this chamber is the
dark ring or iris _i_, which you saw from the front through the cornea
and fluid. Close behind the iris again is the natural 'magic glass' of
our eye, the crystalline lens _l_, which is composed of perfectly
transparent fibres and has two rounded or convex surfaces like an
ordinary magnifying glass. This lens rests on a cushion of a soft
jelly-like substance _v_, called the vitreous humour, which fills the
dark chamber or cavity of the eyeball and keeps it in shape, so that the
retina _r_, which lines the chamber, is kept at a proper distance from
the lens. This retina is a transparent film of very sensitive nerves; it
forms a screen at the back of the chamber, and has a coating of very
dark pigment or colouring matter behind it. Lastly, the nerves of the
retina all meet in a bundle, called the optic nerve, and passing out of
the eyeball at a point _on_, go to the brain. These are the chief parts
we use in seeing; now how do we use them?
"Suppose that a pencil is held in front of the eye at the distance at
which we see small objects comfortably. Light is reflected from all
parts of the surface of the pencil, and as the rays spread, a certain
number enter the pupil of the eye. We will follow only two cones of
light coming from the points 1 and 2 on the diagram Fig. 11. These you
see enter the eye, each widely spread over the cornea _c_. They are bent
in a little by this curved covering, and by the liquid behind it, while
the iris cuts off the rays near the edges of the lens, which would be
too much bent to form a clear image. The rest of the rays fall upon the
lens _l_. In passing through this lens they are very much bent (or
_refracted_) towards each other, so much so that by the time they reach
the end of the dark chamber _v_, each cone of light has come to a point
or focus 1', 2', and as rays of this kind have come from every point all
over the pencil, exactly similar points are formed on the retina, and a
real picture of the pencil is formed there between 1' and 2'."
[Illustration: Fig. 12.
Image of a candle-flame thrown on paper by a lens.]
"We will make a very simple and pretty experiment to illustrate this.
Darkening the room I light a candle, take a square of white paper in my
hand, and hold a simple magnifying glass between the two (see Fig. 12)
about three inches away from the candle. Then I shift the paper nearer
and farther behind the lens, till we get a clear image of the
candle-flame upon it. This is exactly what happens in our eye. I have
drawn a dotted line _c_ round the lens and the paper on the diagram to
represent the eyeball in which the image of the candle-flame would be on
the retina instead of on the piece of paper. The first point you will
notice is that the candle-flame is upside down on the paper, and if you
turn back to Fig. 11 you will see why, for it is plain that the cones of
light _cross_ in the lens _l_, 1 going to 1' and 2 to 2'. Every picture
made on our retina is upside down.
"But it is not there that we see it. As soon as the points of light from
the pencil strike upon the retina, the thrill passes on along the optic
nerve _on_, through the back of the eye to the brain; and our mind,
following back the rays exactly as they have come through the lens, sees
a pencil, outside the eye, right way upwards.
"This is how we see with our eyes, which adjust themselves most
beautifully to our needs. For example, not only is the iris always ready
to expand or contract according as we need more or less light, but there
is a special muscle, called the ciliary muscle (_cm_, Fig. 11), which
alters the lens for us to see things far or near. In all, or nearly all,
perfect eyes the lens is flatter in front than behind, and this enables
us to see things far off by bringing the rays from them exactly to a
focus on the retina. But when we look at nearer things the rays require
to be more bent or refracted, so without any conscious effort on our
part this ciliary muscle contracts and allows the lens to bulge out
slightly in front. Instantly we have a stronger magnifier, and the rays
are brought to the right focus on the retina, so that a clear and
full-size image of the near object is formed. How little we think, as we
turn our eyes from one thing to another, and observe, now the distant
hills, now the sheep feeding close by; or, as night draws on, gaze into
limitless space and see the stars millions upon millions of miles away,
that at every moment the focus of our eye is altering, the iris is
contracting or expanding, and myriads of images are being formed one
after the other in that little dark chamber, through which pass all the
scenes of the outer world!
"Yet even this wonderful eye cannot show us everything. Some see farther
than others, some see more minutely than others, according as the lens
of the eye is flatter in one person and more rounded in another. But the
most long-sighted person could never have discovered the planet Neptune,
more than 2700 millions of miles distant from us, nor could the
keenest-sighted have known of the existence of those minute and
beautiful little plants, called diatoms, which live around us wherever
water is found, and form delicate flint skeletons so infinitesimally
small that thousands of millions go to form one cubic inch of the stone
called tripoli, found at Bilin in Bohemia."
[Illustration: Fig. 13.
Arrow magnified by a convex lens.
_a_, _b_, Real arrow. C, D, Magnifying-glass. A, B, Enlarged image of
the arrow.]
"It is here that our 'magic glasses' come to our assistance, and reveal
to us what was before invisible. We learnt just now that we see near
things by the lens of our eye becoming more rounded in front; but there
comes a point beyond which the lens cannot bulge any more, so that when
a thing is very tiny, and would have to be held very close to the eye
for us to see it, the lens can no longer collect the rays to a focus, so
we see nothing but a blur. More than 800 years ago an Arabian, named
Alhazen, explained why rounded or convex glasses make things appear
larger when placed before the eye. This glass which I hold in my hand
is a simple magnifying-glass, such as we used for focusing the
candle-flame. It bends the rays inwards from any small object (see the
arrow _a_, _b_, Fig. 13) so that the lens of our eye can use them, and
then, as we follow out the rays in straight lines to the place where we
see clearly (at A, B), every point of the object is magnified, and we
not only see it much larger, but every mark upon it is much more
distinct. You all know how the little shilling magnifying-glasses you
carry show the most lovely and delicate structures in flowers, on the
wings of butterflies, on the head of a bee or fly, and, in fact, in all
minute living things."
[Illustration: Fig. 14.
Student's microscope. _ep_, Eye-piece. _o_, _g_, Object-glass.]
[Illustration: Fig. 15.
Skeleton of a microscope, showing how an object is magnified.
_o_, _l_, Object-lens. _e_, _g_, Eye-glass. _s_, _s_, Spicule.
_s'_, _s'_, Magnified image of same in the tube. S, S, Image again
enlarged by the lens of the eye-piece.]
"But this is only our first step. Those diatoms we spoke of just now
will only look like minute specks under even the strongest
magnifying-glass. So we pass on to use two extra lenses to assist our
eyes, and come to this compound microscope (Fig. 14) through which I
have before now shown you the delicate markings on shells which were
themselves so minute that you could not see them with the naked eye. Now
we have to discover how the microscope performs this feat. Going back
again for a minute to our candle and magnifying-glass (Fig. 12), you
will find that the nearer you put the lens to the candle the farther
away you will have to put the paper to get a clear image. When in a
microscope we put a powerful lens _o_, _l_ close down to a very minute
object, say a spicule of a flint sponge _s_, _s_, quite invisible to the
unaided eye, the rays from this spicule are brought to a focus a long
way behind it at _s'_, _s'_, making an enlarged image because the lines
of light have been diverging ever since they crossed in the lens. If you
could put a piece of paper at _s'_ _s'_, as you did in the candle
experiment, you would see the actual image of the magnified spicule upon
it. But as these points of light are only in an empty tube, they pass
on, spreading out again from the image, as they did before from the
spicule. Then another convex lens or eye-glass _e_, _g_ is put at the
top of the microscope at the proper distance to bend these rays so that
they enter our eye in nearly parallel lines, exactly as we saw in the
ordinary magnifying-glass (Fig. 13), and our crystalline lens can then
bring them to a focus on our retina.
"By this time the spicule has been twice magnified; or, in other words,
the rays of light coming from it have been twice bent towards each
other, so that when our eye follows them out in straight lines they are
widely spread, and we see every point of light so clearly that all the
spots and markings on this minute spicule are as clear as if it were
really as large as it looks to us.
"This is simply the principle of the microscope. When you come to look
at your own instruments, though they are very ordinary ones, you will
find that the object-glass _o_, _l_ is made of three lenses, flat on the
side nearest the tube, and each lens is composed of two kinds of glass
in order to correct the unequal refraction of the rays, and prevent
fringes of colour appearing at the edge of the lens. Then again the
eye-piece will be a short tube with a lens at each end, and halfway
between them a black ledge will be seen inside the tube which acts like
the iris of our eye (_i_, Fig. 11) and cuts off the rays passing through
the edges of the lens. All these are devices to correct faults in the
microscope which our eye corrects for itself, and they have enabled
opticians to make very powerful lenses.
"Look now at the diagram (Fig. 16) showing a group of diatoms which you
can see under the microscope after the lecture. Notice the lovely
patterns, the delicate tracery, and the fine lines on the diatoms shown
there. Yet each of these minute flint skeletons, if laid on a piece of
glass by itself, would be quite invisible to the naked eye, while
hundreds of them together only look like a faint mist on the slide on
which they lie. Nor are they even here shown as much magnified as they
might be; under a stronger power we should see those delicate lines on
the diatoms broken up into minute round cups."
[Illustration: Fig. 16.
Fossil diatoms seen under the microscope. The largest of these is an
almost imperceptible speck to the naked eye.]
"Is it not wonderful and delightful to think that we are able to add in
this way to the power of our eyes, till it seems as if there were no
limit to the hidden beauties of the minute forms of our earth, if only
we can discover them?
"But our globe does not stand alone in the universe, and we want not
only to learn all about everything we find upon it, but also to look out
into the vast space around us and discover as much as we can about the
myriads of suns and planets, comets and meteorites, star-mists and
nebulae, which are to be found there. Even with the naked eye we can
admire the grand planet Saturn, which is more than 800 millions of miles
away, and this in itself is very marvellous. Who would have thought that
our tiny crystalline lens would be able to catch and focus rays, sent
all this enormous distance, so as actually to make a picture on our
retina of a planet, which, like the moon, is only sending back to us the
light of the sun? For, remember, the rays which come to us from Saturn
must have travelled twice 800 millions of miles--884 millions from the
sun to the planet, and less or more from the planet back to us,
according to our position at the time. But this is as nothing when
compared to the enormous distances over which light travels from the
stars to us. Even the nearest star we know of, is at least twenty
_millions_ of _millions_ of miles away, and the light from it, though
travelling at the rate of 186,300 miles in a second, takes four years
and four months to reach us, while the light from others, which we can
see without a telescope, is between twenty and thirty years on its road.
Does not the thought fill us with awe, that our little eye should be
able to span such vast distances?
"But we are not yet nearly at the end of our wonder, for the same power
which devised our eye gave us also the mind capable of inventing an
instrument which increases the strength of that eye till we can actually
see stars so far off that their light takes _two thousand years_ coming
to our globe. If the microscope delights us in helping us to see things
invisible without it, because they are so small, surely the telescope is
fascinating beyond all other magic glasses when we think that it brings
heavenly bodies, thousands of billions of miles away, so close to us
that we can examine them."
[Illustration: Fig. 17.
An astronomical telescope.
_ep_, Eye-piece. _og_, Object-glass. _f_, Finder.]
"A Telescope (Fig. 17) can, like the microscope, be made of only two
glasses: an object-glass to form an image in the tube and a magnifying
eye-piece to enlarge it. But there is this difference, that the object
lens of a microscope is put close down to a minute object, so that the
rays fall upon it at a wide angle, and the image formed in the tube is
very much larger than the object outside. In the telescope, on the
contrary, the thing we look at is far off, so that the rays fall on the
object-glass at such a very narrow angle as to be practically parallel,
and the image in the tube is of course _very, very_ much smaller than
the house, or church, or planet it pictures. What the object-glass of
the telescope does for us, is to bring a small _real image_ of an object
very far off close to us in the tube of the telescope so that we can
examine it.
"Think for a moment what this means. Imagine that star we spoke of (p.
41), whose light, travelling 186,300 miles in one second, still takes
2000 years to reach us. Picture the tiny waves of light crossing the
countless billions of miles of space during those two thousand years,
and reaching us so widely spread out that the few faint rays which
strike our eye are quite useless, and for us that star has no existence;
we cannot see it. Then go and ask the giant telescope, by turning the
object-glass in the direction where that star lies in infinite space.
The widespread rays are collected and come to a minute bright image in
the dark tube. You put the eye-piece to this image, and there, under
your eye, is a shining point: this is the image of the star, which
otherwise would be lost to you in the mighty distance.
"Can any magic tale be more marvellous, or any thought grander, or more
sublime than this? From my little chamber, by making use of the laws of
light, which are the same wherever we turn, we can penetrate into depths
so vast that we are not able even to measure them, and bring back unseen
stars to tell us the secrets of the mighty universe. As far as the stars
are concerned, whether we see them or not depends entirely upon the
number of rays collected by the object-glass; for at such enormous
distances the rays have no angle that we can measure, and magnify as
you will, the brightest star only remains a point of light. It is in
order to collect enough rays that astronomers have tried to have larger
and larger object-glasses; so that while a small good hand telescope,
such as you use, may have an object-glass measuring only an inch and a
quarter across, some of the giant telescopes have lenses of two and a
half feet, or thirty inches, diameter. These enormous lenses are very
difficult to make and manage, and have many faults, therefore
astronomical telescopes are often made with curved mirrors to _reflect_
the rays, and bring them to a focus instead of _refracting_ them as
curved lenses do.
"We see, then, that one very important use of the telescope is to bring
objects into view which otherwise we would never see; for, as I have
already said, though we bring the stars into sight, we cannot magnify
them. But whenever an object is near enough for the rays to fall even at
a very small perceptible angle on the object-glass, then we can magnify
them; and the longer the telescope, and the stronger the eye-piece, the
more the object is magnified.
[Illustration: Fig. 18.
Skeletons of telescopes.
A, A one-foot telescope with a three-inch eye-piece. B, A two-foot
telescope with a three-inch eye-piece. _e_, _p_, Eye-piece. _o_, _g_,
Object-glass. _r_, _r_, Rays which enter the telescopes and crossing at
_x_ form an image at _i_, _i_, which is magnified by the lens _e_, _p_.
The angles _r_, _x_, _r_ and _i_, _x_, _i_ are the same. In A the angle
_i_, _o_, _i_ is four times greater than that of _i_, _x_, _i_. In B it
is eight times greater.]
"I want you to understand the meaning of this, for it is really very
simple, only it requires a little thought. Here are skeleton drawings of
two telescopes (Fig. 18), one double the length of the other. Let us
suppose that two people are using them to look at an arrow on a
weathercock a long distance off. The rays of light _r_, _r_ from the
two ends of the arrow will enter both telescopes at the same angle
_r_, _x_, _r_, cross in the lens, and pass on at _exactly the same
angle_ into the tubes. So far all is alike, but now comes the
difference. In the short telescope A the object-glass must be of such a
curve as to bring the cones of light in each ray to a focus at a
distance of _one foot_ behind it,[1] and there a small image _i_, _i_ of
the arrow is formed. But B being twice the length, allows the lens
to be less curved, and the image to be formed _two feet_ behind the
object-glass; and as the rays _r_, _r_ have been _diverging_ ever since
they crossed at _x_, the real image of the arrow formed at _i_, _i_ is
twice the size of the same image in A. Nevertheless, if you could put a
piece of paper at _i_, _i_ in both telescopes, and look through the
_object-glass_ (which you cannot actually do, because your head would
block out the rays), the arrow would appear the same size in both
telescopes, because one would be twice as far off from you as the other,
and the angle _i_, _x_, _i_ is the same in both."
[1] In our Fig. 18 the distances are inches instead of feet, but the
proportions are the same.
"But by going to the proper end of the telescope you can get quite near
the image, and can see and magnify it, if you put a strong lens to
collect the rays from it to a focus. This is the use of the eye-piece,
which in our diagram is placed at a quarter of a foot or three inches
from the image in both telescopes. Now that we are close to the images,
the divergence of the points _i_, _i_ makes a great difference. In the
small telescope, in which the image is only _one foot_ behind the
object-glass, the eye-piece being a quarter of a foot from it, is four
times nearer, so the angle _i_, _o_, _i_ is four times the angle
_i_, _x_, _i_, and the man looking through it sees the image magnified
_four times_. But in the longer telescope the image is _two feet_ behind
the lens, while the eye-piece is, as before, a quarter of a foot from
it. Thus the eyepiece is now eight times nearer, so the angle
_i_, _o_, _i_ is eight times the angle _i_, _x_, _i_, and the observer
sees the image magnified _eight times_.
"In real telescopes, where the difference between the focal length of
the object-glass and that of the eye-glass can be made enormously
greater, the magnifying power is quite startling, only the object-glass
must be large, so as to collect enough rays to bear spreading widely.
Even in your small telescopes, with a focus of eighteen inches, and an
object-glass measuring one and a quarter inch across, we can put on a
quarter of an inch eye-piece, and so magnify seventy-two times; while in
my observatory telescope, eight feet or ninety-six inches long, an
eye-piece of half an inch magnifies 192 times, and I can put on a
1/8-inch eye-piece and magnify 768 times! And so we can go on
lengthening the focus of the object-glass and shortening the focus of
the eye-piece, till in Lord Rosse's gigantic fifty-six-foot telescope,
in which the image is fifty-four feet (648 inches) behind the
object-glass, an eye-piece one-eighth of an inch from the image
magnifies 5184 times! These giant telescopes, however, require an
enormous object-glass or mirror, for the points of light are so spread
out in making the large image that it is very faint unless an enormous
number of rays are collected. Lord Rosse's telescope has a reflecting
mirror measuring six feet across, and a man can walk upright in the
telescope tube. The most powerful telescope yet made is that at the Lick
Observatory, on Mount Hamilton, in California. It is fifty-six and a
half feet long, the object-lens measures thirty-six inches across. A
star seen through this telescope appears 2000 times as bright as when
seen with the naked eye.
"You need not, however, wait for an opportunity to look through giant
telescopes, for my small student's telescope, only four feet long, which
we carry out on to the lawn, will show you endless unseen wonders; while
your hand telescopes, and even a common opera-glass, will show many
features on the face of the moon, and enable you to see the crescent of
Venus, Jupiter's moons, and Saturn's rings, besides hundreds of stars
unseen by the naked eye.
"Of course you will understand that Fig. 18 only shows the _principle_
of the telescope. In all good instruments the lenses and other parts are
more complicated; and in a terrestrial telescope, for looking at
objects on the earth, another lens has to be put in to turn them right
way up again. In looking at the sky it does not matter which way up we
see a planet or a star, so the second glass is not needed, and we lose
light by using it.
"We have now three magic glasses to work for us--the magnifying-glass,
the microscope, and the telescope. Besides these, however, we have two
other helpers, if possible even more wonderful. These are the
Photographic camera and the Spectroscope."
[Illustration: Fig. 19.
Photographic camera.
_l_, _l_, Lenses. _s_, _s_, Screen cutting off diverging rays. _c_, _c_,
Sliding box. _p_, _p_, Picture formed.]
"Now that we thoroughly understand the use of lenses, I need scarcely
explain this photographic camera (Fig. 19), for it is clearly an
artificial eye. In place of the _crystalline lens_ (compare with Fig.
11) the photographer uses one, or generally two lenses _l_, _l_, with a
black ledge or stop _s_ between them, which acts like the iris in
cutting off the rays too near the edge of the lens. The dark camera _c_
answers to the _dark chamber_ of the eyeball, and the plate _p_, _p_ at
the back of the chamber, which is made sensitive by chemicals, answers
our _retina_. The box is formed of two parts, sliding one within the
other at _c_, so as to place the plate at a proper distance from the
lens, and then a screw adjusts the focus more exactly by bringing the
front lens back or forward, instead of altering the curve as the
_ciliary muscle_ does in our eye. The difference between the two
instruments is that in our eye the message goes to the brain, and the
image disappears when we turn our eyes away from the object; but in the
camera the waves of light work upon the chemicals, and the image can be
fixed and remain for ever.
"But the camera has at least one weak point. The screen at the back is
not curved like our retina, but must be flat because of printing off the
pictures, and therefore the parts of the photograph near the edge are a
little out of proportion.
"In many ways, however, this photographic eye is a more faithful
observer than our own, and helps us to make more accurate pictures. For
instance, instantaneous photographs have been taken of a galloping
horse, and we find that the movements are very different from what we
thought we saw with our eye, because our retina does not throw off one
impression after another quickly enough to be quite certain we see each
curve truly in succession. Again, the photograph of a face gives minute
curves and lines, lights and shadows, far more perfectly than even the
best artist can see them, and when the picture is magnified we see more
and more details which escaped us before.
"But it is especially when attached to the microscope or the telescope
that the photographic apparatus tells us such marvellous secrets;
giving us, for instance, an accurate picture of the most minute
water-animal quite invisible to the naked eye, so that when we enlarge
the photograph any one can see the beautiful markings, the finest fibre,
or the tiniest granule; or affording us accurate pictures, such as the
one at p. 19 of the face of the moon, and bringing stars into view which
we cannot otherwise see even with the strongest telescope.
"Our own eye has many weaknesses. For example, when we look through the
telescope at the sky we can only fix our attention on one part at once,
and afterwards on another; and the picture which we see in this way, bit
by bit, we must draw as best we can. But if we put a sensitive
photographic plate into the telescope just at the point (_i_, _i_, Fig.
18), where the _image_ of the sky is focused, this plate gives
attention, so to speak, to the whole picture at once, and registers
every point exactly as it is; and this picture can be kept and enlarged
so that every detail can be seen.
"Then, again, if we look at faint stars, they do not grow any brighter
as we look. Each ray sends its message to the brain, and that is all; we
cannot heap them up in our eye, and, indeed, after a time we see less,
because our nerves grow tired. But on a photographic plate in a
telescope, each ray in its turn does a little work upon the chemicals,
and the longer the plate remains, the stronger the picture becomes. When
wet plates were used they could not be left long, but since dry plates
have been invented, with a film of chemically prepared gelatine, they
can be left for hours in the telescope, which is kept by clockwork
accurately opposite to the same objects. In this way thousands of faint
stars, which we cannot see with the strongest telescope, creep into view
as their feeble rays work over and over again on the same spot; and, as
the brighter stars as well as the faint ones are all the time making
their impression stronger, when the plate comes out each one appears in
its proper strength. On the other hand, very bright objects often become
blurred by a long exposure, so that we have sometimes to sacrifice the
clearness of a bright object in order to print faint objects clearly.
"We now come to our last magic glass--the Spectroscope; and the hour has
slipped by so fast that I have very little time left to speak of it. But
this matters less as we have studied it before.[1] I need now only
remind you of some of the facts. You will remember that when we passed
sunlight through a three-sided piece of glass called a prism, we broke
up a ray of white light into a line of beautiful colours gradually
passing from red, through orange, yellow, green, blue, and indigo, to
violet, and that these follow in the same order as we see them in the
rainbow or in the thin film of a soap-bubble. By various experiments we
proved that these colours are separated from each other because the many
waves which make up white light are of different sizes, so that because
the waves, of red light are slow and heavy, they lag behind when bent in
the three-sided glass, while the rapid violet waves are bent more out of
their road and run to the farther end of the line, the other colours
ranging themselves between."
[1] _Fairyland of Science_, Lecture II.; and _Short History of Natural
Science_, chapter xxxiv.
"Now when the light falls through the open window, or through a round
hole or _large_ slit, the images of the hole made by each coloured wave
overlap each other very much, and the colours in the spectrum or
coloured band are crowded together. But when in the spectroscope we pass
the ray of light through a very narrow slit, each coloured image of the
upright slit overlaps the next upright image only very little. By using
several prisms one after the other (see Fig. 21), these upright coloured
lines are separated more and more till we get a very long band or
spectrum. Yet, as you know from our experiments with the light of a
glowing wire or of molten iron, however much you spread out the light
given by a solid or liquid, you can never separate these coloured lines
from each other. It is only when you throw the light of a glowing gas or
vapour into the slit that you get a few bright lines standing out alone.
This is because _all_ the rays of white light are present in glowing
solids and liquids, and they follow each other too closely to be
separated. But a gas, such as glowing hydrogen for example, gives out
only a few separate rays, which, pouring through the slit, throw red,
greenish-blue, and dark blue lines on the screen. Thus you have seen the
double, orange-yellow sodium line (3, Plate I.) which starts out at once
when salt is held in a flame and its light thrown into the spectroscope,
and the red line of potassium vapour under the same treatment; and we
shall observe these again when we study the coloured lights of the sun
and stars."
[Illustration: Fig. 20.
Kirchhoff's spectroscope.
A, The telescope which receives the ray of light through the slit in O.]
[Illustration: Fig. 21.
Passage of rays through the spectroscope.
S, S', Slit through which the light falls on the prisms. 1, 2, 3, 4,
Prisms in which the rays are dispersed more and more. _a_, _b_, Screen
receiving the spectrum, of which the seven principal colours are
marked.]
"We see, then, that the work of our magic glass, the spectroscope, is
simply to sift the waves of light, and that these waves, from their
colour and their position in the long spectrum, actually tell us what
glowing gases have started them on their road. Is not this like magic?
I take a substance made of I know not what; I break it up, and, melting
it in the intense heat of an electric spark, throw its light into the
spectroscope. Then, as I examine this light after it has been spread out
by the prisms, I can actually read by unmistakable lines what metals or
non-metals it contains. Nay, more; when I catch the light of a star, or
even of a faint nebula, in my telescope, and pass it through these
prisms, there, written up on the magic-coloured band, I read off the
gases which are glowing in that star-sun or star-dust billions of miles
away.
"Now, boys, I have let you into the secrets of my five magic
glasses--the magnifying-glass, the microscope, the telescope, the
photographic camera, and the spectroscope. With these and the help of
chemistry you can learn to work all my spells. You can peep into the
mysteries of the life of the tiniest being which moves unseen under your
feet; you can peer into that vast universe, which we can never visit so
long as our bodies hold us down to our little earth; you can make the
unseen stars print their spots of light on the paper you hold in your
hand, by means of light-waves, which left them hundreds of years ago; or
you can sift this light in your spectroscope, and make it tell you what
substances were glowing in that star when they were started on their
road. All this you can do on one condition, namely, that you seek
patiently to know the truth.
"Stories of days long gone by tell us of true magicians and false
magicians, and the good or evil they wrought. Of these I know nothing,
but I do know this, that the value of the spells you can work with my
magic glasses depends entirely upon whether you work patiently,
accurately, and honestly. If you make careless, inaccurate experiments,
and draw hasty conclusions, you will only do bad work, which it may take
others years to undo; but if you question your instruments honestly and
carefully, they will answer truly and faithfully. You may make many
mistakes, but one experiment will correct the other; and while you are
storing up in your own mind knowledge which lifts you far above this
little world, or enables you to look deep below the outward surface of
life, you may add your little group of facts to the general store, and
help to pave the way to such grand discoveries as those of Newton in
astronomy, Bunsen and Kirchhoff in spectrum analysis, and Darwin in the
world of life."
CHAPTER III
FAIRY RINGS AND HOW THEY ARE MADE
[Illustration]
It was a lovely warm day in September, the golden corn had been cut and
carted, and the waggons of the farmers around were free for the use of
the college lads in their yearly autumn holiday. There they stood in a
long row, one behind the other in the drive round the grounds, each with
a pair of sleek, powerful farm-horses, and the waggoners beside them
with their long whips ornamented with coloured ribbons; and as each
waggon drew up before the door, it filled rapidly with its merry load
and went on its way.
They had a long drive of seven miles before them, for they were going to
cross the wild moor, and then descend gradually along a fairly good road
to the more wooded and fertile country. Their object that day was to
reach a certain fairy dell known to a few only among the party as one of
the loveliest spots in Devon. It was a perfect day for a picnic. As they
drove over the wide stretches of moorland, with tors to right and tors
to the left, the stunted furze bushes growing here and there glistened
with spiders' webs from which the dew had not yet disappeared, and
mosses in great variety carpeted the ground, from the lovely
thread-mosses, with their scarlet caps, to the pale sphagnum of the
bogs, where a halt was made for some of the botanists of the party to
search for the little Sundew (_Drosera rotundifolia_). Though this
little plant had now almost ceased to flower, it was not difficult to
recognise by its rosette of leaves glistening with sticky glands which
it spreads out in many of the Dartmoor bogs to catch the tiny flies and
suck out their life's blood, and several specimens were uprooted and
carefully packed away to plant in moist moss at home.
From this bog onwards the road ran near by one of the lovely streams
which feed the rivers below, and, passing across a bridge covered with
ivy, led through a small forest of stunted trees round which the
woodbine clung, hanging down its crimson berries, and the bracken fern,
already putting on its brown and yellow tints, grew tall and thick on
either side. Then, as they passed out of the wood, they came upon the
dell, a piece of wild moorland lying in a hollow between two granite
ridges, with large blocks of granite strewn over it here and there, and
furze bushes growing under their shelter, still covered with yellow
blossoms together with countless seed-bearing pods, which the
youngsters soon gathered for the shiny-black seeds within them.
Here the waggons were unspanned, the horses tethered out, the food
unpacked, and preparations for the picnic soon in full swing. Just at
this moment, however, a loud shout from one part of the dell called
every one's attention. "The fairy rings! the fairy rings! we have found
the fairy rings!" and there truly on the brown sward might be seen three
delicate green rings, the fresh sprouting grass growing young and tender
in perfect circles measuring from six feet to nearly three yards across.
"What are they?" The question came from many voices at once, but it was
the Principal who answered.
"Why, do you not know that they are pixie circles, where the 'elves of
hills, brooks, standing lakes, and groves' hold their revels, whirling
in giddy round, and making the rings, 'whereof the ewe not bites'? Have
you forgotten how Mrs. Quickly, in the _Merry Wives of Windsor_, tells
us that
"'nightly, meadow-fairies, look you sing,
Like to the Garter's compass, in a ring:
The expressure that it bears, green let it be,
More fertile-fresh than all the field to see'?
"If we are magicians and work spells under magic glasses, why should not
the pixies work spells on the grass? I brought you here to-day on
purpose to see them. Which of you now can name the pixie who makes
them?"
A deep silence followed. If any knew or guessed the truth of the matter,
they were too shy to risk making a mistake.
"Be off with you then," said the Principal, "and keep well away from
these rings all day, that you may not disturb the spell. But come back
to me before we return at night, and perhaps I may show you the
wonder-working pixie, and we may take him home to examine under the
microscope."
The day passed as such happy days do, and the glorious harvest moon had
risen over the distant tors before the horses were spanned and the
waggons ready. But the Principal was not at the starting place, and
looking round they saw him at the farther end of the dell.
"Gently, gently," he cried, as there was one general rush towards him;
"look where you tread, for I stand within a ring of fairies!"
And then they saw that just outside the green circle in which he stood,
forming here and there a broken ring, were patches of a beautiful tiny
mushroom, each of which raised its pale brown umbrella in the bright
moonlight.
"Here are our fairies, boys. I am going to take a few home where they
can be spared from the ring, and to-morrow we will learn their history."
* * * * *
The following day saw the class-room full, and from the benches eager
eyes were turned to the eight windows, in each of which stood one of the
elder boys at his microscope ready for work. For under those microscopes
the Principal always arranged some object referred to in his lecture and
figured in diagrams on the walls, and it was the duty of each boy, after
the lecture was over, to show and explain to the class all the points
of the specimen under his care. These boys were always specially envied,
for though the others could, it is true, follow all the descriptions
from the diagrams, yet these had the plant or animal always under their
eye. Discussion was at this moment running high, for there was a great
uncertainty of opinion as to whether a mushroom could be really called a
plant when it had no leaves or flowers. All at once the hush came, as
the Principal stepped into his desk and began:--
"Life is hard work, boys, and there is no being in this world which has
not to work for its living. You all know that a plant grows by taking in
gases and water, and working them up into sap and living tissue by the
help of the sunshine and the green matter in their leaves; and you know,
too, that the world is so full of green plants that hundreds and
thousands of young seedlings can never get a living, but are stifled in
their babyhood or destroyed before they can grow up.
"Now there are many dark, dank places in the world where plants cannot
get enough sunlight and air to make green colouring matter and
manufacture their own food. And so it comes to pass that a certain class
of plants have found another way of living, by taking their food ready
made from other decaying plants and animals, and so avoiding the
necessity of manufacturing it for themselves. These plants can live
hidden away in dark cellars and damp cupboards, in drains and pipes
where no light ever enters, under a thick covering of dead leaves in the
forest, under fallen trunks and mossy stones; in fact, wherever
decaying matter, whether of plant or animal, can be found for them to
feed upon.
"It is to this class, called _fungi_, which includes all mushrooms and
moulds, mildews, smuts, and ferments, that the mushroom belongs which we
found yesterday making the fairy rings. And, in truth, we were not so
far wrong when we called them pixies or imps, for many of them are
indeed imps of mischief, which play sorry pranks in our stores at home
and in the fields and forest abroad. They grow on our damp bread, or
cheese, or pickles; they destroy fruit and corn, hop and vine, and even
take the life of insects and other animals. Yet, on the other hand, they
are useful in clearing out unhealthy nooks and corners, and purifying
the air; and they can be made to do good work by those who know how to
use them; for without ferments we could have neither wine, beer, nor
vinegar, nor even the yeast which lightens our bread.
"I am going to-day to introduce you to this large vagabond class of
plants, that we may see how they live, grow, and spread, what good and
bad work they do, and how they do it. And before we come to the
mushrooms, which you know so well, we must look at the smaller forms,
which do all their work above ground, so that we can observe them. For
the _fungi_ are to be found almost everywhere. The film growing over
manure-heaps, the yeast plant, the wine fungus, and the vinegar plant;
the moulds and mildews covering our cellar-walls and cupboards, or
growing on decayed leaves and wood, on stale fruit, bread, or jam, or
making black spots on the leaves of the rose, the hop, or the vine; the
potato fungus, eating into the potato in the dark ground and producing
disease; the smut filling the grains of wheat and oats with disease, the
ergot feeding on the rye, the rust which destroys beetroot, the rank
toadstools and puffballs, the mushroom we eat, and the truffles which
form even their fruit underground,--all these are _fungi_, or lowly
plants which have given up making their own food in the sunlight, and
take it ready made from the dung, the decaying mould, the root, the
leaf, the fruit, or the germ on which they grow. Lastly, the diseases
which kill the silkworm and the common house-fly, and even some of the
worst skin diseases in man, are caused by minute plants of this class
feeding upon their hosts."
[Illustration: Fig. 22.
Three forms of vegetable mould magnified.
1, _Mucor Mucedo_. 2, _Aspergillus glaucus_. 3, _Penicillium glaucum_.]
"In fact, the _fungi_ are so widely spread over all things living and
dead, that there is scarcely anything free from them in one shape or
another. The minute spores, now of one kind, now of another, float in
the air, and settling down wherever they find suitable food, have
nothing more to do than to feed, fatten, and increase, which they do
with wonderful rapidity. Let us take as an example one of the moulds
which covers damp leaves, and even the paste and jam in our cupboard. I
have some here growing upon a basin of paste, and you see it forms a
kind of dense white fur all over the surface, with here and there a
bluish-green tinge upon it. This white fur is the common mould, _Mucor
Mucedo_ (1, Fig. 22), and the green mould happens in this case to be
another mould, _Penicillium glaucum_ (3, Fig. 22); but I must warn you
that these minute moulds look very much alike until you examine them
under the microscope, and though they are called white, blue, or green
moulds, yet any one of them may be coloured at different times of its
growth. Another very common and beautiful mould, _Aspergillus glaucus_
(2, Fig. 22), often grows with Mucor on the top of jam.
"All these plants begin with a spore or minute colourless cell of living
matter (_s_, Fig. 23), which spends its energy in sending out tubes in
all directions into the leaves, fruit, or paste on which it feeds. The
living matter, flowing now this way now that, lays down the walls of its
tubes as it flows, and by and by, here and there, a tube, instead of
working into the paste, grows upwards into the air and swells at the tip
into a colourless ball in which a number of minute seed-like bodies
called spores are formed. The ball bursts, the spores fall out, and each
one begins to form fresh tubes, and so little by little the mould grows
denser and thicker by new plants starting in all directions.
"Under the first microscope you will see a slide showing the tubes which
spread through the paste, and which are called the _mycelium_ (_m_, Fig.
23), and amongst it are three upright tubes, one just starting _a_,
another with the fruit ball forming _b_, and a third _c_, which is
bursting and throwing out the spores. The _Aspergillus_ and the
_Penicillium_ differ from the _Mucor_ in having their spores naked and
not enclosed in a spore-case. In _Penicillium_ they grow like the beads
of a necklace one above the other on the top of the upright tube, and
can very easily be separated (see Fig. 22); while _Aspergillus_, a most
lovely silvery mould, is more complicated in the growth of its spores,
for it bears them on many rows branching out from the top of the tube
like the rays of a star."
[Illustration: Fig. 23.
_Mucor Mucedo_, greatly magnified. (After Sachs and Brefeld.)
_m_, Mycelium, or tangle of threads. _a_, _b_, _c_, Upright tubes in
different stages. _c_, Spore-case bursting and sending out spores. _s_,
1, 2, 3, A growing spore, in different stages, starting a new mycelium.]
"I want you to look at each of these moulds carefully under the
microscope, for few people who hastily scrape a mould away, vexed to
find it on food or damp clothing, have any idea what a delicate and
beautiful structure lies under their hand. These moulds live on decaying
matter, but many of the mildews, rusts, and other kinds of fungus, prey
upon living plants such as the _smut_ of oats (_Ustilago carbo_), and
the _bunt_ (_Tilletia caria_) which eats away the inside of the grains
of wheat, while another fungus attacks its leaves. There is scarcely a
tree or herb which has not one fungus to prey upon it, and many have
several, as, for example, the common lime-tree, which is infested by
seventy-four different fungi, and the oak by no less than 200.
"So these colourless food-taking plants prey upon their neighbours,
while they take their oxygen for breathing from air. The 'ferments,'
however, which live _inside_ plants or fluids, take even their oxygen
for breathing from their hosts.
"If you go into the garden in summer and pluck an overripe gooseberry,
which is bursting like this one I have here, you will probably find that
the pulp looks unhealthy and rotten near the split, and the gooseberry
will taste tart and disagreeable. This is because a small fungus has
grown inside, and worked a change in the juice of the fruit. At first
this fungus spread its tubes outside and merely _fed_ upon the fruit,
using oxygen from the air in breathing; but by and by the skin gave way,
and the fungus crept inside the gooseberry where it could no longer get
any fresh air. In this dilemma it was forced to break up the sugar in
the fruit and take the oxygen out of it, leaving behind only alcohol and
carbonic acid which give the fermented taste to the fruit.
"So the fungus-imp feeds and grows in nature, and when man gets hold of
it he forces it to do the same work for a useful purpose, for the
grape-fungus grows in the vats in which grapes are crushed and kept away
from air, and tearing up the sugar, leaves alcohol behind in the
grape-juice, which in this way becomes wine. So, too, the yeast-fungus
grows in the malt and hop liquor, turning it into beer; its spores
floating in the fluid and increasing at a marvellous rate, as any
housewife knows who, getting yeast for her bread, tries to keep it in a
corked bottle.
"The yeast plant has never been found wild. It is only known as a
cultivated plant, growing on prepared liquor. The brewer has to sow it
by taking some yeast from other beer, or by leaving the liquor exposed
to air in which yeast spores are floating; or it will sow itself in the
same way in a mixture of water, hops, sugar, and salt, to which a
handful of flour is added. It increases at a marvellous rate, one cell
budding out of another, while from time to time the living matter in a
cell will break up into four parts instead of two, and so four new cells
will start and bud. A drop of yeast will very soon cover a glass slide
with this tiny plant, as you will see under the second microscope, where
they are now at work (Fig. 24)."
[Illustration: Fig. 24.
Yeast cells growing under the microscope. _a_, Single cells. _b_, Two
cells forming by division. _c_, A group of cells where division is going
on in all directions.]
"But perhaps the most curious of all the minute fungi are those which
grow inside insects and destroy them. At this time of year you may
often see a dead fly sticking to the window-pane with a cloudy white
ring round it; this poor fly has been killed by a little fungus called
_Empusa muscae_. A spore from a former plant has fallen perhaps on the
window-pane, or some other spot over which the fly has crawled, and
being sticky has fixed itself under the fly's body. Once settled on a
favourable spot it sends out a tube, and piercing the skin of the fly,
begins to grow rapidly inside. There it forms little round cells one
after the other, something like the yeast-cells, till it fills the whole
body, feeding on its juices; then each cell sends a tube, like the
upright tubes of the _Mucor_ (Fig. 23) out again through the fly's skin,
and this tube bursts at the end, and so new spores are set free. It is
these tubes, and the spores thrown from them, which you see forming a
kind of halo round the dead fly as it clings to the pane. Other fungi in
the same way kill the silkworm and the caterpillars of the cabbage
butterfly. Nor is it only the lower animals which suffer. When we once
realise that fungus spores are floating everywhere in the air, we can
understand how the terrible microscopic fungi called _bacteria_ will
settle on an open wound and cause it to fester if it is not properly
dressed.
"Thus we see that these minute fungi are almost everywhere. The larger
ones, on the contrary, are confined to the fields and forests, damp
walls and hollow trees; or wherever rotting wood, leaves, or manure
provide them with sufficient nourishment. Few people have any clear
ideas about the growth of a mushroom, except that the part we pick
springs up in a single night. The real fact is, that a whole mushroom
plant is nothing more than a gigantic mould or mildew, only that it is
formed of many different shaped cells, and spreads its tubes
_underground_ or through the trunks of trees instead of in paste or jam,
as in the case of the mould."
[Illustration: Fig. 25.
Early stages of the mushroom. (After Sachs.)
_m_, Mycelium. _b1-3_, Mushroom buds of different ages. _b4_, Button
mushroom. _g_, Gills forming inside before lower attachment of the cap
gives way at _v_.]
"The part which we gather and call a mushroom, a toadstool, or a
puffball is only the fruit, answering to the round balls of the mould.
The rest of the plant is a thick network of tubes, which you will see
under the third microscope. These tubes spread underground and suck in
decayed matter from the earth; they form the _mycelium_ (_m_, Fig. 25)
such as we found in the mould. The mushroom-growers call it 'mushroom
spawn' because they use it to spread over the ground for new crops. Out
of these underground tubes there springs up from time to time a swollen
round body no bigger at first than a mustard seed (_b1_, Fig. 25). As it
increases in size it comes above ground and grows into the mushroom or
spore-case, answering to the round balls which contain the spores of the
mould. At first this swollen body is egg-shaped, the top half being
largest and broadest, and the fruit is then called a 'button-mushroom'
_b4_. Inside this ball are now formed a series of folds made of long
cells, some of which are soon to bear spores just as the tubes in the
mould did, and while these are forming and ripening, a way out is
preparing for them. For as the mushroom grows, the skin of the lower
part of the ball (_v_, _b4_) is stretched more and more, till it can
bear the strain no longer and breaks away from the stalk; then the ball
expands into an umbrella, leaving a piece of torn skin, called the veil
(_v_, Fig. 26), clinging to the stalk."
[Illustration: Fig. 26.
Later stages of the mushroom. (After Gautier.)
1, Button mushroom stage. _c_, Cap. _v_, Veil. _g_, Gills.
2, Full-grown mushroom, showing veil v after the cap is quite free, and
the gills or lamellae _g_, of which the structure is shown in Fig. 27.]
"All this happens in a single night, and the mushroom is complete, with
a stem up the centre and a broad cap, under which are the folds which
bear the spores. Thus much you can see for yourselves at any time by
finding a place where mushrooms grow and looking for them late at night
and early in the morning so as to get the different stages. But now we
must turn to the microscope, and cutting off one of the folds, which
branch out under the cap like the spokes of a wheel, take a slice across
it (1, Fig. 27) and examine."
[Illustration: Fig. 27.
1, One of the gills or lamellae of the mushroom slightly magnified,
showing the cells round the edge. _c_, Cells which do not bear spores.
_fc_, Fertile cells. 2, A piece of the edge of the same powerfully
magnified, showing how the spores _s_ grow out of the tip of the fertile
cells _fc_.]
"First, under a moderate power, you will see the cells forming the
centre of the fold and the layer of long cells (_c_ and _fc_) which are
closely packed all round the edge. Some of these cells project beyond
the others, and it is they which bear the spores. We see this plainly
under a very strong power when you can distinguish the sterile cells _c_
and the fertile cells _fc_ projecting beyond them, and each bearing
four spore-cells _s_ on four little horns at its tip.
"These spores fall off very easily, and you can make a pretty experiment
by cutting off a large mushroom head in the early morning and putting it
flat upon a piece of paper. In a few hours, if you lift it very
carefully, you will find a number of dark lines on the paper, radiating
from a centre like the spokes of a wheel, each line being composed of
the spores which have fallen from a fold as it grew ripe. They are so
minute that many thousands would be required to make up the size of the
head of an ordinary pin, yet if you gather the spores of the several
kinds of mushroom, and examine them under a strong microscope, you will
find that even these specks of matter assume different shapes in the
various species.
"You will be astonished too at the immense number of spores contained in
a single mushroom head, for they are reckoned by millions; and when we
remember that each one of these is the starting point of a new plant, it
reminds us forcibly of the wholesale destruction of spores and seeds
which must go on in nature, otherwise the mushrooms and their companions
would soon cover every inch of the whole world.
"As it is, they are spread abroad by the wind, and wherever they escape
destruction they lie waiting in every nook and corner till, after the
hot summer, showers of rain hasten the decay of plants and leaves, and
then the mushrooms, toadstools, and puffballs, grow at an astounding
pace. If you go into the woods at this season you may see the enormous
deep-red liver fungus (_Fistulina hepatica_) growing on the oak-trees,
in patches which weigh from twenty to thirty pounds; or the glorious
orange-coloured fungus (_Tremella mesenterica_) growing on bare sticks
or stumps of furze; or among dead leaves you may easily chance on the
little caps of the crimson, scarlet, snowy white, or orange-coloured
fungi which grow in almost every wood. From white to yellow, yellow to
red, red to crimson and purple black, there is hardly any colour you may
not find among this gaily-decked tribe; and who can wonder that the
small bright-coloured caps have been supposed to cover tiny imps or
elves, who used the large mushrooms to serve for their stools and
tables?
"There they work, thrusting their tubes into twigs and dead branches,
rotting trunks and decaying leaves, breaking up the hard wood and tough
fibres, and building them up into delicate cells, which by and by die
and leave their remains as food for the early growing plants in the
spring. So we see that in their way the mushrooms and toadstools are
good imps after all, for the tender shoot of a young seedling plant
could take no food out of a hard tree-trunk, but it finds the work done
for it by the fungus, the rich nourishment being spread around its young
roots ready to be imbibed.
"To find our fairy-ring mushrooms, however, we must leave the wood and
go out into the open country, especially on the downs and moors and
rough meadows, where the land is poor and the grass coarse and spare.
There grow the nourishing kinds, most of which we can eat, and among
these is the delicate little champignon or 'Scotch-bonnet' mushroom,
_Marasmius Oreades_,[1] which makes the fairy-rings. When a spore of
this mushroom begins to grow, it sucks up vegetable food out of the
earth and spreads its tubes underground, in all directions from the
centre, so that the mycelium forms a round patch like a thick
underground circular cobweb. In the summer and autumn, when the weather
is suitable, it sends up its delicate pale-brown caps, which we may
gather and eat without stopping the growth of the plant.
[1] Shown in initial letter of this chapter.
"This goes on year after year underground, new tubes always travelling
outwards till the circle widens and widens like the rings of water on a
pond, only that it spreads very slowly, making a new ring each year,
which is often composed of a mass of tubes as much as a foot thick in
the ground, and the tender tubes in the centre die away as the new ones
form a larger hoop outside.
"But all this is below ground; where then are our fairy rings? Here is
the secret. The tubes, as we have seen, take up food from the earth and
build it up into delicate cells, which decay very soon, and as they die
make a rich manure at the roots of the grass. So each season the cells
of last year's ring make a rich feeding-ground for the young grass,
which springs up fresh and green in a fairy ring, while _outside_ this
emerald circle the mushroom tubes are still growing and increasing
underneath the grass, so that next year, when the present ring is no
longer richly fed, and has become faded and brown like the rest of the
moor, another ring will spring up outside it, feeding on the prepared
food below."
"In bad seasons, though the tubes go on spreading and growing below, the
mushroom fruit does not always appear above ground. The plant will only
fruit freely when the ground has been well warmed by the summer sun,
followed by damp weather to moisten it. This gives us a rich crop of
mushrooms all over the country, and it is then you can best see the ring
of fairy mushrooms circling outside the green hoop of fresh grass. In
any case the early morning is the time to find them; it is only in very
sheltered spots that they sometimes last through the day, or come up
towards evening, as I found them last night on the warm damp side of the
dell.
"This is the true history of fairy rings, and now go and look for
yourselves under the microscopes. Under the first three you will find
the three different kinds of mould of our diagram (Fig. 22). Under the
fourth the spores of the mould are shown in their first growth putting
out the tubes to form the mycelium. The fifth shows the mould itself
with its fruit-bearing tubes, one of which is bursting. Under the sixth
the yeast plant is growing; the seventh shows a slice of one of the
folds of the common mushroom with its spore-bearing horns; and under the
eighth I have put some spores from different mushrooms, that you may see
what curious shapes they assume.
"Lastly, let me remind you, now that the autumn and winter are coming,
that you will find mushrooms, toadstools, puffballs, and moulds in
plenty wherever you go. Learn to know them, their different shapes and
colours, and above all the special nooks each one chooses for its home.
Look around in the fields and woods and take note of the decaying plants
and trees, leaves and bark, insects and dead remains of all kinds. Upon
each of these you will find some fungus growing, breaking up their
tissues and devouring the nourishing food they provide. Watch these
spots, and note the soft spongy soil which will collect there, and then
when the spring comes, notice what tender plants flourish upon these
rich feeding grounds. You will thus see for yourselves that the fungi,
though they feed upon others, are not entirely mischief-workers, but
also perform their part in the general work of life."
CHAPTER IV
THE LIFE-HISTORY OF LICHENS AND MOSSES
[Illustration]
The autumn has passed away and we are in the midst of winter. In the
long winter evenings the stars shine bright and clear, and tempt us to
work with the telescope and its helpmates the spectroscope and
photographic plates. But at first sight it would seem as though our
microscopes would have to stand idle so far at least as plants are
concerned, or be used only to examine dried specimens and mounted
sections. Yet this is not the fact, as I remembered last week when
walking through the bare and leafless wood. A startled pheasant rising
with a whirr at the sound of my footsteps among the dead leaves roused
me from my thoughts, and as a young rabbit scudded across the path and I
watched it disappear among the bushes, I was suddenly struck with the
great mass of plant life flourishing underfoot and overhead.
Can you guess what plants these were? I do not mean the evergreen pines
and firs, nor the few hardy ferns, nor the lovely ivy clothing the
trunks of the trees. Such plants as these live and remain green in the
winter, but they do not grow. If you wish to find plant life revelling
in the cold damp days of winter, fearing neither frost nor snow and
welcoming mist and rain, you must go to the mosses, which as autumn
passes away begin to cover the wood-paths, to creep over the roots of
the trees, to suck up the water in the bogs, and even to clothe dead
walls and stones with a soft green carpet. And with the mosses come the
lichens, those curious grey and greenish oddities which no one but a
botanist would think of classing among plants.
The wood is full of them now: the hairy lichens hang from the branches
of many of the trees, making them look like old greybearded men; the
leafy lichens encircle the branches, their pale gray, green, and yellow
patches looking as if they were made of crumpled paper cut into wavy
plates; and the crusty lichens, scarcely distinguishable from the bark
of the trees, cover every available space which the mosses have left
free.
As I looked at these lichens and thought of their curious history I
determined that we would study them to-day, and gathered a basketful of
specimens (see Fig. 28). But when I had collected these I found I had
not the heart to leave the mosses behind. I could not even break off a
piece of bark with lichen upon it without some little moss coming too,
especially the small thread-mosses (_Bryum_) which make a home for
themselves in every nook and corner of the branches; while the
feather-mosses, hair-mosses, cord-mosses, and many others made such a
lovely carpet under my feet that each seemed too beautiful to pass by,
and they found their way into my basket, crowned at the top with a large
mass of the pale-green Sphagnum, or bog-moss, into which I sank more
than ankle-deep as I crossed the bog in the centre of the wood on my way
home.
[Illustration: Fig. 28.
Examples of Lichens. (From life.)
1, A hairy lichen. 2, A leafy lichen. 3, A crustaceous lichen. _f_, _f_,
the fruit.]
So here they all are, and I hope by the help of our magic glass to let
you into some of the secrets of their lives. It is true we must study
the structure of lichens chiefly by diagrams, for it is too minute for
beginners to follow under the microscope, so we must trust to drawings
made by men more skilful in microscopic botany, at any rate for the
present. But the mosses we can examine for ourselves and admire their
delicate leaves and wonderful tiny spore-cases.
Now the first question which I hope you want to ask is, how it is that
these lowly plants flourish so well in the depth of winter when their
larger and stronger companions die down to the ground. We will answer
this first as to the lichens, which are such strange uncanny-looking
plants that it is almost difficult to imagine they are alive at all; and
indeed they have been a great puzzle to botanists.
[Illustration: Fig. 29.
Single-celled green plants growing and dividing (_Pleurococcus_). (After
Thuret and Bornet.)]
Before we examine them, however, look for a minute at a small drop of
this greenish film which I have taken from the rain-water taken outside.
I have put some under each microscope, and those who can look into them
will see the slide almost covered with small round green cells very much
like the yeast cells we saw when studying the Fungi, only that instead
of being colourless they are a bright green. Some of these cells will I
suspect be longer than others, and these long cells will be moving over
the slide very rapidly, swimming hither and thither, and you will see,
perhaps for the first time, that very low plants can swim about in
water. These green cells are, indeed, the simplest of all plants, and
are merely bags of living matter which, by the help of the green
granules in them, are able to work up water and gases into nourishing
food, and so to live, grow, and multiply.
There are many kinds of these single-celled plants in the world. You may
find them on damp paths, in almost any rain-water butt, in ponds and
ditches, in sparkling waterfalls, along the banks of flowing rivers, and
in the cold clear springs on the bleak mountains. Some of them take the
form of tangled threads[1] composed of long strings of cells, and these
sometimes form long streamers in flowing water, and at other times are
gathered together in a shapeless film only to be disentangled under a
microscope. Other kinds[2] wave to and fro on the water, forming dense
patches of violet, orange-brown, or glossy green scum shining in the
bright sunlight, and these flourish equally in the ponds of our gardens
and in pools in the Himalaya mountains, 18,000 feet above the sea.
Others again[3] seize on every damp patch on tree trunks, rocks, or
moist walls, covering them with a green powder formed of single plant
cells. Other species of this family turn a bright red colour when the
cells are still; and one, under the name of Gory Dew,[4] has often
frightened the peasants of Italy, by growing very rapidly over damp
walls and then turning the colour of blood. Another[5] forms the "red
snow" of the Arctic regions, where it covers wide surfaces of snow with
a deep red colour. Others[6] form a shiny jelly over rocks and stones,
and these may be found almost everywhere, from the garden path to the
warm springs of India, from the marshes of New Zealand up to the shores
of the Arctic ocean, and even on the surface of floating icebergs.
[1] _Confervae._
[2] _Oscillariae._
[3] _Protococcus._
[4] _Palmella cruenta._
[5] _Protococcus nivalis._
[6] _Nostoc._
The reason why these plants can live in such very different regions is
that they do not take their food through roots out of the ground, but
suck in water and gases through the thin membrane which covers their
cell, and each cell does its own work. So it matters very little to them
where they lie, so long as they have moisture and sunlight to help them
in their work. Wherever they are, if they have these, they can take in
carbonic acid from the air and work up the carbon with other gases which
they imbibe with the water, and so make living material. In this way
they grow, and as a cell grows larger the covering is stretched and part
of the digested food goes to build up more covering membrane, and by and
by the cell divides into two and each membrane closes up, so that there
are two single-celled plants where there was only one before. This will
sometimes go on so fast that a small pond may be covered in a few hours
with a green film formed of new cells.
Now we have seen, when studying mushrooms, that the one difference
between these green plants and the single-celled Fungi is that while the
green cells make their own food, colourless cells can only take it in
ready-made, and therefore prey upon all kinds of living matter. This is
just what happens in the lichens; and botanists have discovered that
these curious growths are really the result of a _partnership_ between
single-celled green plants and single-celled fungi. The grey part is a
fungus; but when it is examined under the microscope we find it is not a
fungus only; a number of green cells can be seen scattered through it,
which, when carefully studied, prove to be some species of the green
single-celled plants.
Here are two drawings of sections cut through two different lichens, and
enormously magnified so that the cells are clearly seen. 1, Fig. 30 is
part of a hairy lichen (1, Fig. 28), and 2 is part of a leafy lichen (2,
Fig. 28). The hairy lichen as you see has a row of green cells all round
the tiny branch, with fungus cells on all sides of them. The leafy
lichen, which only presents one surface to the sun and air while the
other side is against the tree, has only one layer of green cells near
the surface, but protected by the fungus above.
[Illustration: Fig. 30.
Sections of Lichens. (Sachs.)
1, Section of a hairy lichen, _Usnea barbata_. 2, Section of a leafy
lichen, _Sticta fuliginosa_. 3, Early growth of a lichen. _gc_, Green
cells. _f_, Fungus.]
The way the lichen has grown is this. A green cell (_gc_ 3, Fig. 30)
falling on some damp spot has begun to grow and spread, working up food
in the sunlight. To it comes the spore of the fungus _f_, first
thrusting its tubes into the tree-bark, or wall, and then spreading
round the green cells, which remain always in such a position that
sunlight, air, and moisture can reach them. From this time the two
classes of plants live as friends, the fungus using part of the food
made by the green cells, and giving them in return the advantage of
being spread out to the sunlight, while they are also protected in
frosty or sultry weather when they would dry up on a bare surface. On
the whole, however, the fungus probably gains the most, for it has been
found, as we should expect, that the green cells can live and grow if
separated out of the lichen, but the fungus cells die when their
industrious companions are taken from them.
At any rate the partnership succeeds, as you will see if you go into the
wood, or into an orchard where the apple-trees are neglected, for every
inch of the branches is covered by lichens if not already taken up by
mosses or toadstools.
There is hardly any part of the world except the tropics where lichens
do not abound. In the Alps of Scandinavia close to the limits of
perpetual snow, in the sandy wastes of Arctic America, and over the
dreary Tundras of Arctic Siberia, where the ground is frozen hard during
the greater part of the year, they flourish where nothing else can live.
The little green cells multiply by dividing, as we saw them doing in the
green film from the water-butt. The fungus, however, has many different
modes of seeding itself. One of these is by forming little pockets in
the lichen, out of which, when they burst, small round bodies are
thrown, which cover the lichen with a minute green powder. There is
plenty of this powder on the leafy lichen which you have by you. You can
see it with the magnifying-glass, without putting it under the
microscope. As long as the lichen is dry these round bodies do not grow,
but as soon as moisture reaches them they start away and become new
plants.
[Illustration: Fig. 31.
Fructification of a lichen. (From Sachs and Oliver.)
Apothecium or spore-chamber of a lichen. 1, Closed. 2, Open. 3, The
spore-cases and filaments enlarged, showing the spores. _f_, Filaments.
_sc_, Spore-cases. _s_, Spores.]
A more complicated and beautiful process is shown in this diagram (Fig.
31). If you look carefully at the leafy lichen (2, Fig. 28) you will
find here and there some little cups _f_, while others grow upon the
tips of the hairy lichen. These cups, or fruits, were once closed,
flask-shaped chambers (1, Fig. 31) inside which are formed a number of
oval cells _sc_, which are spore-cases, with from four to eight spores
or seed-like bodies _s3_ inside them. When these chambers, which are
called _apothecia_, are ripe, moist or rainy weather causes them to
swell at the top, and they burst open and the spore-cases throw out the
spores to grow into new fungi.
In some lichens the chambers remain closed and the spores escape through
a hole in the top, and they are then called _perithecia_, while in
others, as these which we have here, they open out into a cup-shape.
This, then, is the curious history of lichens; the green cells and fungi
flourishing together in the damp winter and bearing the hardest frost
far better than the summer drought, so that they have their good time
when most other plants are dead or asleep. Yet though some of them, such
as the hairy lichens, almost disappear in the summer, they are by no
means dead, for, like all these very low plants, they can bear being
dried up for a long time, and then, when moisture visits them again,
each green cell sets to work, and they revive. There is much more to be
learnt about them, but this will be sufficient to make you feel an
interest in their simple lives, and when you look for them in the wood
you will be surprised to find how many different kinds there are, for it
is most wonderful that such lowly plants should build up such an immense
variety of curious and grotesque forms.
* * * * *
And yet, when we turn to the mosses, I am half afraid they will soon
attract you away from the dull grey lichens, for of all plant histories
it appears to me that the history of the moss-plant is most fascinating.
As this history is complicated by the moss having, as it were, two
lives, you must give me your whole attention, and I will explain it
first from diagrams, though you can see all the steps under the
microscope.
Take in your hands, in the first place, a piece of this green moss which
I have brought. How thick it is, like a rich felted carpet! and yet, if
you pull it apart carefully, you will find that each leafy stem is
separate, and can be taken away from the others without breaking
anything. In this dense moss each stem is single and clothed with leaves
wrapped closely round it (see Fig. 33); in some mosses the stem is
branched, and in others the leaves grow on side stalks, as in this
feathery moss (Fig. 32). But in each case every stem is like a separate
plant, with its own tuft of tender roots _r_.
[Illustration: Fig. 32.
A stem of feathery moss. (From life.)
_l_, Leaves. _s_, Stem. _r_, Roots.]
What a delicate growth it is! The stem is scarcely more than a fine
thread, the leaves minute, transparent, and tender. In this pale
sphagnum or bog-moss (Fig. 36, p. 93), which is much larger and stouter,
you can see better how each one of these leaves, though they are so
thickly packed, is placed so that it can get the utmost light, air, and
moisture. Yet so closely are the leaves of each stem entangled in those
of the next that the whole forms a thick springy green carpet under our
feet.
How is it, then, that these moss stems, though each independent, grow in
such a dense mass? Partly because moss multiplies so rapidly that new
stems are always thrusting themselves up to the light, but chiefly
because the stems were not always separate, but in very early life
sprang from a common source.
If, instead of bringing the moss home and tearing it apart, you went to
a spot in the wood where fresh moss was growing, and looked very
carefully on the surface of the ground or among the water of a marsh,
you would find a spongy green mass below the growing moss, very much
like the green scum on a pond. This film, some of which I have brought
home, is seen under the microscope to be a mass of tangled green threads
(_t_, Fig. 34, p. 88) like those of the _Confervae_ (see p. 79), composed
of rows of cells, while here and there upon these threads you would find
a bud (_mb_, Fig. 34) rising up into the air.
This tangled mass of green threads, called the _protonema_, is the first
growth, from which the moss stems spring. It has itself originated from
a moss-spore; as we shall see by and by. As soon as it has started it
grows and spreads very rapidly, drinking in water and air through all
its cells and sending up the moss buds which swell and grow, giving out
roots below and fine stems above, which soon become crowded with leaves,
forming the velvety carpet we call moss. Meanwhile the soft threads
below die away, giving up all their nourishment to the moss-stems, and
this is why, when you take up the moss, you find each stem separate. But
now comes the question, How does each stem live after the nourishing
threads below have died? It is true each stem has a few hairy roots,
but these are very feeble, and not at all like the roots of higher
plants. The fact is, the moss is built up entirely of tender cells, like
the green cells in the lichen, or in the film upon the pond. These cells
are not shut in behind a thick skin as in the leaves of higher plants,
but have every one of them the power to take in water and gases through
their tender membrane.
I made last night a rough drawing of the leaf of the feathery moss put
under the microscope, but you will see it far better by putting a leaf
with a little water on a glass slide under the covering glass and
examining it for yourself. You will see that it is composed of a number
of oval-shaped cells packed closely together (_c_ Fig. 33), with a few
long narrow ones _mr_ in the middle of the leaf forming the midrib.
Every cell is as clear and distinct as if it were floating in the water,
and the tiny green grains which help it to work up its food are clearly
visible.
[Illustration: Fig. 33.
Moss-leaf magnified. (From life.)
Showing the cells _c_, each of which can take in and work up its own
food. _mr_, Long cells of the mid-rib.]
Each of these cells can act as a separate plant, drinking in the water
and air it needs, and feeding and growing quite independently of the
roots below. Yet at the same time the moss stem has a great advantage
over single-celled plants in having root-hairs, and being able to grow
upright and expose its leaves to the sun and air.
Now you will no longer wonder that moss grows so fast and so thick, and
another curious fact follows from the independence of each cell, namely,
that new growths can start from almost any part of the plant. For
example, pieces will often break off from the tangled mass or protonema
below, and, starting on their own account, form other thread masses.
Then, after the moss stems have grown, a new mass of threads may grow
from one of the tiny root-hairs of a stem and make a fresh tangle; nay,
a thread will sometimes even spring out of a damp moss leaf and make a
new beginning, while the moss stems themselves often put forth buds and
branches, which grow root-hairs and settle down on their own account.
[Illustration: Fig. 34.
Polytrichum commune. A large hair-moss.
_t_, _t_, Threads of green cells forming the _protonema_ out of which
moss-buds spring. _mb_, Buds of moss-stems. _a_, Minute green flower in
which the antherozoids are formed (enlarged in Fig. 35). _p_, _p1_,
_p2_, _p3_, Minute green flower in which the ovules are formed, and
urn-plant springing out of it (enlarged in Fig. 35). _us_, Urn stems.
_c_, Cap. _u_, Urn after cap has fallen off, still protected by its
lid.]
All this comes from the simple nature of the plants, each cell doing its
own work. Nor are the mosses in any difficulty as to soil, for as the
matted threads decay they form a rich manure, and the dying moss-stems
themselves, being so fragile, turn back very readily into food. This is
why mosses can spread over the poorest soil where even tough grasses
cannot live, and clothe walls and roofs with a rich green.
[Illustration: Fig. 35.
Fructification of a moss.
A, Male moss-flower stripped of its outer leaves, showing jointed
filaments and oval sacs os and antherozoid cells _zc_ swarming out of a
sac. _zc'_, Antherozoid cell enlarged. _z_, Free antherozoid. P, Female
flower with bottle-shaped sacs _bs_. _bs-c_, Bottle-shaped sac, with cap
being pushed up. _u_, Urn of _Funaria hygrometrica_, with small cap.
_u'_, Urn, from which the cap has fallen, showing the teeth _t_ which
keep in the spores.]
So far, then, we now understand the growth of the mossy-leaf stems, but
this is only half the life of the plant. After the moss has gone on
through the damp winter spreading and growing, there appear in the
spring or summer tiny moss flowers at the tip of some of the stems.
These flowers (_a_, _p_, Fig. 34) are formed merely of a few green
leaves shorter and stouter than the rest, enclosing some oval sacs
surrounded by jointed hairs or filaments (see A and P, Fig. 35). These
sacs are of two different kinds, one set being short and stout _os_, the
others having long necks like bottles _bs_. Sometimes these two kinds of
sac are in one flower, but more often they are in separate flowers, as
in the hair-moss, _Polytrichum commune_ (_a_ and _p_, Fig. 34). Now when
the flowers are ripe the short sacs in the flower A open and fling out
myriads of cells _zc_, and these cells burst, and forth come tiny
wriggling bodies _z_, called by botanists _antherozoids_, one out of
each cell. These find their way along the damp moss to the flower P, and
entering the neck of one of the bottle-shaped sacs _bs_, find out each
another cell or _ovule_ inside. The two cells together then form a
_plant-egg_, which answers to the germ in the seeds of higher plants.
Now let us be sure we understand where we are in the life of the plant.
We have had its green-growing time, its flowering, and the formation of
what we may roughly call its seed, which last in ordinary higher plants
would fall down and grow into a new green plant. But with the moss there
is more to come. The egg does not shake out of the bottle-necked sac,
but begins to grow inside it, sending down a little tube into the moss
stem, and using it as other plants use the ground to grow in.
As soon as it is rooted it begins to form a delicate stem, and as this
grows it pushes up the sac _bs_, stretching the neck tighter and tighter
till at last it tears away below, and the sac is carried up and hangs
like an extinguisher or cap (_c_ Figs. 34, 35) over the top of the stem.
Meanwhile, under this cap the top of the stalk swells into a knob
which, by degrees, becomes a lovely little covered urn _u_, something
like a poppy head, which has within it a number of spores. The growth of
this tiny urn-plant often occupies several months, for you must remember
that it is not merely a fruit, though it is often called so, but a real
plant, taking in food through its tubes below and working for its
living.
When it is finished it is a most lovely little object (_us_, Fig. 34),
the fine hairlike stalk being covered with a green, yellow, or brilliant
red fool's cap on the top, yet the whole in most mosses is not bigger
than an ordinary pin. You may easily see them in the spring or summer,
or even sometimes in the winter. I have only been able to bring you one
very little one to-day, the _Funaria hygrometrica_, which fruits early
in the year. This moss has only a short cap, but in many mosses they are
very conspicuous. I have often pulled them off as you would pull a cap
from a boy's head. In nature they fall off after a time, leaving the
urn, which, though so small, is a most complicated structure. First it
has an outer skin, with holes or mouths in it which open and close to
let moisture in and out. Then come two layers of cells, then an open
space full of air, in which are the green chlorophyll grains which are
working up food for the tiny plant as the moisture comes in to them.
Lastly, within this again is a mass of tissue, round which grow the
spores which are soon to be sown, and which in _Polytrichum commune_ are
protected by a lid. Even after the extinguisher and the lid have both
fallen off, the spores cannot fall out, for a thick row of teeth (_t_,
Fig. 35) is closed over them like the tentacles of an anemone. So long
as the air is damp these teeth remain closed; it is only in fine dry
weather that they open and the spores are scattered on the ground.
_Funaria hygrometrica_ has no lid under its cap, and after the cap falls
the spores are only protected by the teeth.
When the spores are gone, the life of the tiny urn-plant is over. It
shrivels and dies, leaving ten, fifteen, or even more spores, which,
after lying for some time on the ground, sprout and grow into a fresh
mass of soft threads.
So now we have completed the life-history of the moss and come back to
the point at which we started. I am afraid it has been rather a
difficult history to follow step by step, and yet it is perfectly clear
when once we master the succession of growths. Starting from a spore,
the thread-mass or protonema gives rise to the moss-stems forming the
dense green carpet, then the green flowers on some of the leaf-stems
give rise to a plant-egg, which roots itself in the stem, and grows into
a perfect plant without leaves, bearing merely the urn in which fresh
spores are formed, and so the round goes on from year to year.
There are a great number of different varieties of moss, and they differ
in the shape and arrangement of their stems and leaves, and very much in
the formation of their urns, yet this sketch will enable you to study
them with understanding, and when you find in the wood the nodding caps
of the fruiting plants, some red, some green, some yellow, and some a
brilliant orange, you will feel that they are acquaintances, and by the
help of the microscope may soon become friends.
Among them one of the most interesting is the sphagnum or bog-moss (Fig.
36), which spreads its thick carpet over all the bogs in the woods. You
cannot miss its little orange-coloured spore-cases if you look closely,
for they contrast strongly with its pale green leaves, out of which they
stand on very short stalks. I wish we could examine it, for it differs
much from other mosses, both in leaves and fruit, but it would take us
too long. At least, however, you must put one of its lovely transparent
leaves under the microscope, that you may see the large air-cells which
lie between the growing cells, and admire the lovely glistening bands
which run across and across their covering membrane, for the sphagnum
leaf is so extremely beautiful that you will never forget it when once
seen. It is through these large cells in the edge of the stem and leaf
that the water rises up from the swamp, so that the whole moss is like a
wet sponge.
[Illustration: Fig. 36.
Sphagnum moss from a Devonshire bog. (From life.)]
And now, before we part, we had better sum up the history of lichens and
mosses. With the lichens we have seen that the secret of success seems
to be mutual help. The green cells provide the food, the fungus cells
form a surface over which the green cells can spread to find sunlight
and moisture, and protection from extremes of heat or cold. With the
mosses the secret lies in their standing on the borderland between two
classes of plant life. On the one hand, they are still tender-celled
plants, each cell being able to live its own life and make its own food;
on the other hand, they have risen into shapely plants with the
beginnings of feeble roots, and having stems along which their leaves
are arranged so that they are spread to the light and air. Both lichens
and mosses keep one great advantage common to all tender-celled plants;
they can be dried up so that you would think them dead, and yet, because
they can work all over their surface whenever heat and moisture reach
them, each cell drinks in food again and the plant revives. So when a
scorching sun, or a dry season, or a biting frost kills other plants,
the mosses and lichens bide their time till moisture comes again.
In our own country they grow almost everywhere--on walls, on broken
ground, on sand-heaps, on roofs and walls, on trees living and dead, and
over all pastures which are allowed to grow poor and worn out. They
grow, too, in all damp, marshy spots; especially the bog-mosses forming
the peat-bogs which cover a large part of Ireland and many regions in
Scotland; and these same bog-mosses occur in America, New Zealand, and
Australia.
In the tropics mosses are less abundant, probably because other plants
flourish so luxuriantly; but in Arctic Siberia and Arctic America both
lichens and mosses live on the vast Tundras. There, during the three
short months of summer, when the surface of the ground is soft, the
lichens spread far and wide where all else is lifeless, while in moister
parts the Polytrichums or hair-mosses cover the ground, and in swampy
regions stunted Sphagnums form peat-bogs only a few inches in depth over
the frozen soil beneath. If, then, the lichens and mosses can flourish
even in such dreary latitudes as these, we can understand how they defy
even our coldest winters, appearing fresh and green when the snow melts
away from over them, and leave their cells bathed in water, so that
these lowly plants clothe the wood with their beauty when otherwise all
would be bare and lifeless.
CHAPTER V
THE HISTORY OF A LAVA STREAM
[Illustration]
It is now just twenty-two years ago, boys, since I saw a wonderful
sight, which is still so fresh in my mind that I have to look round and
remember that it was before any of you were born, in order to persuade
myself that it can be nearly a quarter of a century since I stood with
my feet close to a flowing stream of red-hot lava.
It happened in this way. I was spending the winter with friends in
Naples, and we were walking quietly one lovely afternoon in November
along the Villa Reale, the public garden on the sea-shore, when one of
our party exclaimed, "Look at Vesuvius!" We did so, and saw in the
bright sunlight a dense dark cloud rising up out of the cone. The
mountain had been sending out puffs of smoke, with a booming noise, for
several days, but we thought nothing of that, for it had been common
enough for slight eruptions to take place at intervals ever since the
great eruption of 1867. This cloud, however, was far larger and
wider-spread than usual, and as we were looking at it we saw a thin red
line begin some way down the side of the mountain and creep onwards
toward the valley which lies behind the Hermitage near where the
Observatory is built (see Fig. 37). "A crater has broken out on the
slope," said our host; "it will be a grand sight to-night. Shall we go
up and see it?" No sooner proposed than settled, and one of the party
started off at once to secure horses and men before others engaged them.
[Illustration: Fig. 37.
Somma. Vesuvius.
Vesuvius, as seen in eruption by the author, November 1868.]
It was about eight o'clock in the evening when we started in a carriage
for Resina, and alighting there, with buried Herculaneum under our feet,
mounted our horses and set forward with the guides. Then followed a long
ascent of about two hours and a half through the dark night. Silently
and carefully we travelled on over the broad masses of slaggy lava of
former years, along which a narrow horse-path had been worn; and ever
and anon we heard the distant booming in the crater at the summit, and
caught sight of fresh gleams of light as we took some turning which
brought the glowing peak into view.
Our object was to get as close as possible to the newly-opened crater in
the mountain-side, and when we arrived on a small rugged plain not far
from the spot, we alighted from our horses, which were growing
frightened with the glare, and walked some distance on foot till we came
to a ridge running down the slope, and upon this ridge the lava stream
was flowing.
Above our heads hung a vast cloud of vapour which reflected the bright
light from the red-hot stream, and threw a pink glow all around, so
that, where the cloud was broken and we could see the dark sky, the
stars looked white as silver in contrast. We could now trace clearly the
outline of the summit towering above us, and even watch the showers of
ashes and dust which burst forth from time to time, falling back into
the crater, or on to the steep slopes of the cone.
If the night had not been calm, and such a breeze as there was blowing
away from us, our position would scarcely have been safe; and indeed we
were afterwards told we had been rash. But I would have faced even a
greater risk to see so grand a spectacle, and when the guide helped me
to scramble up on to the ledge, so that I stood with my feet within a
few yards of the lava flow, my heart bounded with excitement. I could
not stay more than a few seconds, for the gases and vapour choked me;
but for that short time it felt like a dream to be standing close to a
river of molten rock, which a few hours before had been lying deep in
the bowels of the earth. Glancing upwards to where this river issued
from the cone in the mountain-side, I saw it first white-hot, then
gradually fading to a glowing red as it crept past my feet; and then
looking down the slope I saw it turn black and gloomy as it cooled
rapidly at the top, while through the cracks which opened here and there
as it moved on, puffs of gas and vapour rose into the air, and the red
lava beneath gleamed through the chinks.
We did not stay long, for the air was suffocating, but took our way back
to the Hermitage, where another glorious sight awaited us. Some way
above and behind the hill on which the Observatory stands there is, or
was, a steep cliff, and over this the lava stream, now densely black,
fell in its way to the valley below, and as it fell it broke into huge
masses, which heeling over exposed the red-hot lava under the crust,
thus forming a magnificent fiery cascade in which black and red were
mingled in wild confusion.
This is how I saw a fresh red-hot lava stream. I had ascended the
mountain some years before, when it was comparatively quiet, with only
two small cones in its central crater sending out miniature flows of
lava (see Fig. 38). But the crater was too hot for me to cross over to
these cones, and I could only marvel at the mass of ashes of which the
top of the mountain was composed, and plunge a stick into an old lava
stream to see how hot it still remained below. Peaceful and quiet as the
mountain seemed then, I could never have imagined such a glorious
outburst as that of November 1868 unless I had seen it, and yet this was
quite a small eruption compared to those of 1867 and 1872, which in
their turn were nothing to some of the older eruptions in earlier
centuries.
[Illustration: Fig. 38.
The top of Vesuvius in 1864. (After Nasmyth.)]
Now it is the history of this lava stream which I saw, that we are going
to consider to-day, and you will first want to know where it came from,
and what caused it to break out on the mountain-side. The truth is, that
though we know now a good deal about volcanoes themselves, we know very
little about the mighty cauldrons deep down in the earth from which
they come. Our deepest mines only reach to a depth of a little more than
half a mile, and no borings even have been made beyond three-quarters of
a mile, so that after this depth we are left very much to guesswork.
We do know that the temperature increases as we go farther down from the
surface, but the increase is very different in different districts--in
some places being five times greater than it is in others at an equal
depth, and it is always greatest in localities where volcanoes have been
active not long before. Now if there were an ocean of melted rock at a
certain distance down below the crust all over the globe, there could
scarcely be such a great difference between one place and another, and
for this and many other reasons geologists are inclined to think that,
from some unknown cause, great heat is developed at special points below
the surface at different times. This would account for our finding
volcanic rocks in almost all parts of the world, even very far away from
where there are any active volcanoes now.
But, as I have said, we do not clearly know why great reservoirs of
melted rock occur from time to time deep under our feet. We may perhaps
one day find the clue from the fact that nearly all, if not all,
volcanoes occur near to the water's edge, either on the coast of the
great oceans or of some enormous inland sea or lake. But at present all
we can say is, that in certain parts of the globe there must be from
time to time great masses of rock heated till they are white-hot, and
having white-hot water mingled with them. These great masses need not,
however, be liquid, for we know that under enormous pressure white-hot
metals remain solid, and water instead of flashing into steam is kept
liquid, pressing with tremendous force upon whatever keeps it confined.
But now suppose that for some reason the mass of solid rock and ground
above one of these heated spots should crack and become weak, or that
the pressure from below should become so great as to be more powerful
than the weight above, then the white-hot rock and water quivering and
panting to expand, would upheave and burst the walls of their prison.
Cannot you picture to yourselves how when this happened the rock would
swell into a liquid state, and how the water would force its way upwards
into cracks and fissures expanding into steam as it went. Then would be
heard strange rumbling noises underground, as all these heavily
oppressed white-hot substances upheaved and rent the crust above them.
And after a time the country round, or the ground at the bottom of the
sea, would quake and tremble, till by and by a way out would be found,
and the water flashing into vapour would break and fling up the masses
of rock immediately above the passage it had made for itself, and
following after these would come the molten rock pouring out at the new
opening.
Such outbursts as these have been seen at sea many times near volcanic
islands. In 1811 a new island called Sabrina was thrown up off St.
Michael's in the Azores, and after remaining a short time was washed
away by the waves. In the same way Graham's Island appeared off the
coast of Sicily in 1831, and as late as 1885 Mr. Shipley saw a
magnificent eruption in the Pacific near the Tonga Islands when an
island about three miles long was formed.
Another very extraordinary outburst, this time on land, took place in
1538 on the opposite side of the Bay of Naples to where Vesuvius stands.
There, on the shores of the Bay of Baiae, a mountain 440 feet high was
built up in one week, where all had before been quiet in the memory of
man. For two years before the outburst came, rumblings and earthquakes
had alarmed the people, and at last one day the sea drew back from the
shore and the ground sank about fourteen feet, and then on the night of
Sunday, September 29, 1538, it was hurled up again, and steam, fiery
gases, stones, and mud burst forth, driving away the frightened people
from the village of Puzzuoli about two miles distant. For a whole week
jets of lava, fragments of rock, and showers of ashes were poured out,
till they formed the hill now called Monte Nuovo, 440 feet high and
measuring a mile and a half round the base. And there it has remained
till the present day, perfectly quiet after the one great outburst had
calmed down, and is now covered with thickets of stone-pine trees.
These sudden outbursts show that some great change must occur in the
state of the earth's crust under the spots where they take place, and we
know that eruptions may cease for centuries in any particular place and
then begin afresh quite unexpectedly. Vesuvius was a peaceable mountain
overgrown with trees and vines in the time of the Greeks till in the
year A.D. 79 occurred the terrific outburst which destroyed Herculaneum
and Pompeii, shattering old Vesuvius to pieces, so that only the cliffs
on the northwest remain and are called Somma (see Fig. 37), while the
new Vesuvius has grown up in the lap, as it were, of its old self. Yet
when we visit the cliffs of Somma, and examine the old lava streams in
them, we see that the ancient peaceful mountain was itself built up by
volcanic outbursts of molten rock, and showers of clinkers or scoriae,
long before man lived to record it.
Meanwhile, when once an opening is made, we can understand how after an
eruption is over, and the steam and lava are exhausted, all quiets down
for awhile, and the melted rock in the crater of the mountain cools and
hardens, shutting in once more the seething mass below. This was the
state of the crater when I saw it in 1864, though small streams still
flowed out of two minute cones; but since then at least one great
outburst had taken place in 1867, and now on this November night, 1868,
the imprisoned gases rebelled once more and forced their way through the
mountain-side.
At this point we can leave off forming conjectures and really study what
happens; for we do know a great deal about the structure of volcanoes
themselves, and the history of a lava-flow has been made very clear
during the last few years, chiefly by the help of the microscope and
chemical experiments. If we imagine then that on the day of the eruption
we could have seen the inside of the mountain, the diagram (Fig. 39)
will fairly represent what was taking place there.
[Illustration: Fig. 39.
Diagrammatic section of an active volcano.
_a_, Central pipe or funnel. _b_, _b_, Walls of the crater or cup.
_c_, _c_, Dark turbid cloud formed by the ascending globular clouds
_d_, _d_. _e_, Rain-shower from escaped vapour. _f_, _f_, Shower of
blocks, cooled bombs, stones, and ashes falling back on to the cone.
_g_, Lava escaping through a fissure, and pouring out of a cone opened
in the mountain side.]
In the funnel _a_ which passes down from the crater or cup _b_, _b_,
white-hot lava was surging up, having a large quantity of water and
steam entangled in it. The lava, or melted rock, would be in much the
same state as melted iron-slag is, in the huge blast-furnaces in which
iron-rock is fused, only it would have floating in it great blocks of
solid rock, and rounded stones called bombs which have been formed from
pieces of half-melted rock whirled in air and falling back into the
crater, together with clinkers or scoriae, dust and sand, all torn off
and ground down from the walls of the funnel up which the rush was
coming. And in the pipe of melted rock, forcing the lava upwards,
enormous bubbles of steam and gas _d_, _d_ would be rising up one after
another as bubbles rise in any thick boiling substances, such as boiling
sugar or tar.
In the morning before the eruption, when only a little smoke was issuing
from the crater, these bubbles rose very slowly through the loaded
funnel and the half-cooled lava in the basin, and the booming noise,
like that of heavy cannon, heard from time to time, was caused by the
bursting of one of these globes of steam at the top of the funnel, as it
brought up with it a feeble shower of stones, dust, and scoriae.
Meanwhile the lava surging below was forcing a passage _g_ for itself in
a weak part of the mountain-side and, just at the time when our
attention was called to Vesuvius, the violent pressure from below rent
open a mouth or crater at _h_, so that the lava began to flow down the
mountain in a steady stream. This, relieving the funnel, enabled the
huge steam bubbles _d_, _d_ to rise more quickly, and to form the large
whitish-grey cloud _c_, into which from time to time the red-hot blocks,
scoriae, and pumice were thrown up by the escaping steam and gases. These
blocks and fragments then fell back again in a fiery shower _f_, _f_
either into the cup, to be thrown up again by the bursting of the next
bubble, or on to the sides of the cone, making it both broader and
higher.
Only one feature in the diagram was fortunately absent the evening we
went up, namely, the rain-shower _e_. The night, as I said, was calm,
and the air dry, and the steam floated peacefully away. The next night,
however, when many people hurried down from Rome to see the sight they
were woefully disappointed, for rain-showers fell heavily from the
cloud, bringing down with them the dust and ashes, which covered the
unfortunate sight-seers.
This was what happened during the eruption, and the result after a few
days was that the cone was a little higher, with a fresh layer of rough
slaggy scoriae on its slopes, and that on the side of the mountain behind
the Hermitage a new lava stream was added to the many which have flowed
there of late years. What then can we learn from this stream about the
materials which come up out of the depths of the earth, and of the
manner in which volcanic rocks are formed?
The lava as I saw it when coming first out of the newly-opened crater
is, as I have said, like white-hot iron slag, but very soon the top
becomes black and solid, a hard cindery mass full of holes and cavities
with rough edges, caused by the steam and sulphur and other gases
breaking through it.[1] In fact, there are so many holes and bubbles in
it that it is very light and floats on the top of the heavier lava
below, falling over it on to the mountain-side when it comes to the end
of the stream. Still, however, the great mass moves on, so that the
stream slides over these fallen clinkers or scoriae. Thus after an
eruption a new flow consists of three layers; at the top the cooled and
broken crust of clinkers, then the more solid lava, which often remains
hot for years, and lastly another cindery layer beneath, formed of the
scoriae which have fallen from above (see Fig. 40).
[1] For the cindery nature of the surface of such a stream see the
initial letter of this chapter.
[Illustration: Fig. 40.
Section of a lava-flow. (J. Geikie.)
1, Slaggy crust, formed chiefly of scoriae of a glassy nature. 2, Middle
portion where crystals form. 3, Slaggy crust which has slipped down and
been covered by the flow.]
You would be surprised to see how quickly the top hardens, so that you
can actually walk across a stream of lava a day or two after it comes
out from the mountain. But you must not stand still or your shoes would
soon be burnt, and if you break the crust with a stick you will at once
see the red-hot lava below; while after a few days the cavities become
filled with crystals of common salt, sulphur or soda, as the vapour and
gases escape.
Then as time goes on the harder minerals gradually crystallise out of
the melted mass, and iron-pyrites, copper-sulphate, and numerous other
forms of crystal appear in the lower part of the stream. In the clinkers
above, where the cooling goes on very rapidly, the lavas formed are
semi-transparent and look much like common bottle-glass. In fact, if you
take this piece of obsidian or volcanic glass in your hand, you might
think that it had come out of an ordinary glass manufactory and had
nothing remarkable in it.
[Illustration: Fig. 41.
A slice of volcanic glass showing the lines of crystallites and
microliths which are the beginnings of crystals.[1] (J. Geikie.)]
[1] This arrangement in lines is called _fluidal structure_ in lava.
But the microscope tells another tale. I have put a thin slice under the
first microscope, and this diagram (Fig. 41) shows what you will see.
Nothing, you say, but a few black specks and some tiny dark rods. True,
but these specks and rods are the first beginnings of crystals forming
out of the ground-mass of glassy lava as it cools down. They are not
real crystals, but the first step toward them, and by a careful
examination of glassy lavas which have cooled at different rates, they
have been seen under the microscope in all stages of growth, gradually
building up different crystalline forms. When we remember how rapidly
the top of many glassy lavas cool down we can understand that they have
often only time to grow very small.
[Illustration: Fig. 42.
A slice of volcanic glass under the microscope, showing well-developed
microliths. (After Cohen.)]
The smaller specks are called _crystallites_, the rods are called
_microliths_.[1] Under the next microscope you can see the microliths
much more distinctly (Fig. 42) and observe that they grow in very
regular shapes.
[1] _Micros_, little; _lithos_, stone.
Our first slice, however (Fig. 41), tells us something more of their
history, for the fact that they are arranged in lines shows that they
have grown while the lava was flowing and carrying them along in
streams. You will notice that each one has its greatest length in the
direction of the lines, just as pieces of stick are carried along
lengthways in a river. In the second specimen (Fig. 42) the microliths
are much larger and the stream has evidently not been flowing fast, for
they lie in all directions.
This is what we find in the upper part of the stream, but if we look at
a piece of underlying lava we find that it is much more coarse-grained,
and the magnifying-glass shows many crystals in it, as well as a number
of microliths. For this lava, covered by the crust above, has remained
very hot for a long time, and the crystals have had time to build
themselves up out of the microliths and crystallites.
Still there is much glassy groundwork even in these lavas. If we want to
find really stony masses such as porphyry and granite made up entirely
of crystals we must look inside the mountain where the molten rock is
kept intensely hot for long periods, as for example in the fissure _g_,
Fig. 39.
Such fissures sometimes open out on the surface like the one I saw, and
sometimes only penetrate part of the way through the hill; but in either
case when the lava in them cools down, it forms solid walls called dykes
which help to bind the loose materials of the mountain together. We
cannot, of course, examine these in an active volcano, but there are
many extinct volcanoes which have been worn and washed by the weather
for centuries, so that we can see the inside. The dykes laid bare in the
cliffs of Somma are old fissures filled with molten rock which has
cooled down, and they show us many stony lavas; and Mr. Judd tells us of
one beautiful example of a ruined volcano which composes the whole
island of Mull in the Hebrides, where such dykes can be traced right
back to a centre. This centre must once have been a mass of melted
matter far down in the earth, and as you trace the dykes back deeper and
deeper into it, the rocks grow more and more stony, till at last they
are composed entirely of large crystals closely crowded together
without any glassy matter between them. You know this crystalline
structure well, for we have plenty of blocks of granite scattered about
on Dartmoor, showing that at some time long ago molten matter must have
been at work in the depths under Devonshire.
We see then that we can trace the melted rock of volcanoes right
back--from the surface of the lava stream which cools quickly at the
top, hurrying the crystallites and microliths along with it--down
through the volcano to the depths of the earth, where the perfect
crystals form slowly and deliberately in the underground lakes of
white-hot rock which are kept in a melted state at an intense heat.
[Illustration: Fig. 43.
A piece of Dartmoor Granite, drawn from a specimen.]
But I promised you that we would have no guesswork here, and you will
perhaps ask how I can be certain what was going on in the depths when
these crystals were formed. A few years ago I could not have answered
you, but now chemists, and especially two eminent French chemists, MM.
Fouque and Levy, have actually _made_ lavas and shown us how it is done
in Nature.
By using powerful furnaces and bellows they have succeeded in getting
temperatures of all degrees, from a dazzling white heat down to a dull
red, and to keep any temperature they like for a long time, so as to
imitate the state of a mass of melted rock at different depths in the
earth, and in this way they have actually _made_ lavas in their
crucibles. For example, there is a certain whitish rock common in
Vesuvius called _leucotephrite_,[1] which is made up chiefly of crystals
of the minerals called leucite, Labrador felspar, and augite. This they
proposed to make artificially, so they took proper quantities of silica,
alumina, oxide of iron, lime, potash, and soda, and putting them in a
crucible, melted them by keeping them at a white heat. Then they lowered
the temperature to an orange-heat, that is a heat sufficient to melt
steel. They kept this heat for forty-eight hours, after which they took
out some of the mixture and, letting it cool, examined a slice under the
microscope. Within it they found crystals of _leucite_ already formed,
showing that these are the first to grow while the melted rock is still
intensely hot. The rest of the mixture they kept red-hot, or at the
melting-point of copper, for another forty-eight hours, and when they
took it out and examined it they found that the whole of it had been
transformed into microliths of the two other forms of crystals, Labrador
felspar and augite, except some small eight-sided crystals of magnetite
and picotite which are also found in the natural rock.
[1] _Leucos_, white; _tephra_, ashes.
There is no need for you to remember all these names. What I do want you
to remember is, that, at the different temperatures, the right crystals
and beginnings of crystals grew up to form the rock which is found in
Vesuvius. And what is still more interesting, they grew exactly to the
same stages as in the natural rock, which is composed of _crystals_ of
leucite and _microliths_ of the two other minerals.
This is only one among numerous experiments by which we have learnt how
volcanic rocks are formed and at what heat the crystals of different
substances grow. We are only as yet at the beginning of this new study,
and there is plenty for you boys to do by and by when you grow up. Many
experiments have failed as yet to imitate certain rocks, and it is
remarkable that these are usually rocks of very ancient eruptions, when
_perhaps_ our globe may have been in a different state to what it is
now; but this remains for us to find out.
Meanwhile I have still another very interesting slide to show you which
tells us something of what is going on below the volcano. Under the
third microscope I have put a slice of volcanic glass (Fig. 44) in which
you will see really large crystals with dark bands curving round them.
These crystals have clearly not been formed in the glass while the lava
was flowing, first because they are too large to have grown up so
rapidly, and secondly because they are broken at the edges in places and
sometimes partly melted. They have evidently come up with the lava as it
flowed out of the mountain, and the dark bands curving round them are
composed of microliths which have been formed in the flow and have swept
round them, as floating straws gather round a block of wood in a stream.
Such crystals as these are often found in lava streams, and in fact they
make a great difference in the rate at which a stream flows, for a
thoroughly melted lava shoots along at a great pace and often travels
several miles in a very short time; but an imperfectly melted lava full
of crystals creeps slowly along, and often does not travel far from the
crater out of which it flows.
[Illustration: Fig. 44.
Slice of volcanic glass under the microscope, showing large included
crystals brought up from inside the volcano in the fluid lava. The dark
bands are lines of microliths formed as the lava cooled. (J. Geikie.)]
So you see we have proof in this slice of volcanic glass of two separate
periods of crystallisation--the period when the large crystals grew in
the liquid mass under the mountain, and the period when the microliths
were formed after it was poured out above ground. And as we know that
different substances form their crystals at very different temperatures,
it is not surprising that some should be able to take up the material
they require and grow in the underground lakes of melted matter, even
though the rest of the lava was sufficiently fluid to be forced up out
of the mountain.
And here we must leave our lava stream. The microscope can tell us yet
more, of marvellous tiny cavities inside the crystals, millions in a
single inch, and of other crystals inside these, all of which have their
history; but this would lead us too far. We must be content for the
present with having roughly traced a flow of lava from the depths below,
where large crystals form in subterranean darkness, to the open air
above, where we catch the tiny beginnings of crystals hardened into
glassy lava before they have time to grow further.
If you will think a little for yourselves about these wonderful
discoveries made with the magic-glass, you will see how many questions
they suggest to us about the minerals which we find buried in the earth
and running through it in veins, and you will want to know something
about the more precious crystals, such as rubies, diamonds, sapphires,
and garnets, and many others which Nature forms far away out of our
sight. All these depend, though indirectly, upon the strange effects of
underground heat, and if you have once formed a picture in your minds of
what must have been going on before that magnificent lava stream crept
down the mountain-side and added its small contribution to the surface
of the earth, you will study eagerly all that comes in your way about
crystals and minerals, and while you ask questions with the spectroscope
about what is going on in the sun and stars millions of miles away, you
will also ask the microscope what it has to tell of the work going on at
depths many miles under your feet.
CHAPTER VI
AN HOUR WITH THE SUN
[Illustration]
Before beginning upon the subject of our lecture to-day I want to tell
you the story of a great puzzle which presented itself to me when I was
a very young child. I happened to come across a little book--I can see
it now as though it were yesterday--a small square green book called
_World without End_, which had upon the cover a little gilt picture of a
stile with trees on each side of it. That was all. I do not know what
the book was about, indeed I am almost sure I never opened it or saw it
again, but that stile and the title "World without End" puzzled me
terribly. What was on the other side of the stile? If I could cross over
it and go on and on should I be in a world which had no ending, and what
would be on the other side? But then there could be no other side if it
was a world without any end. I was very young, you must remember, and I
grew confused and bewildered as I imagined myself reaching onwards and
onwards beyond that stile and never, never resting. At last I consulted
my greatest friend, an old man who did the weeding in my father's
garden, and whom I believed to be very wise. He looked at first almost
as bewildered as I was, but at last light dawned upon him. "I tell you
what it is, Master Arthur," said he, "I do not rightly know what happens
when there is no end, but I do know that there is a mighty lot to be
found out in this world, and I'm thinking we had better learn first all
about that, and perhaps it may teach us something which will help us to
understand the other."
I daresay you will wonder what this anecdote can have to do with a
lecture on the sun--I will tell you. Last night I stood on the balcony
and looked out far and farther away into the star-depths of the midnight
sky, marvelling what could be the history of those countless suns of
which we see ever more and more as we increase the power of our
telescopes, or catch the faint beams of those we cannot see and make
them print their image on the photographic plate. And, as I grew
oppressed at the thought of this never-ending expanse of suns and at my
own littleness, I remembered all at once the little square book of my
childish days with its gilt stile, and my old friend's advice to learn
first all we can of that which lies nearest.
So to-day, before we travel away to the stars, we had better inquire
what is known about the one star in the heavens which is comparatively
near to us, our own glorious sun, which sends us all our light and heat,
causes all the movements of our atmosphere, draws up the moisture from
the ground to return in refreshing rain, ripens our harvests, awakens
the seeds and sleeping plants into vigorous growth, and in a word
sustains all the energy and life upon our earth. Yet even this star,
which is more than a million times as large as our earth, and bound so
closely to us that a convulsion on its surface sends a thrill right
through our atmosphere, is still so far off that it is only by
questioning the sunbeams it sends to us, that we can know anything about
it.
You have already learnt[1] a good deal as to the size, the intense heat
and light, and the photographic power of the sun, and also how his white
beams of light are composed of countless coloured rays which we can
separate in a prism. Now let us pass on to the more difficult problem of
the nature of the sun itself, and what we know of the changes and
commotions going on in that blazing globe of light.
[1] _Fairyland of Science_, Chapter II.
We will try first what we can see for ourselves. If you take a card and
make a pin-hole in it, you can look through this hole straight at the
sun without injuring your eye, and you will see a round shining disc on
which, perhaps, you may detect a few dark spots. Then if you take your
hand telescopes, which I have shaded by putting a piece of smoked glass
inside the eye-piece, you will find that this shining disc is really a
round globe, and moreover, although the object-glass of your telescopes
measures only two-and-a-half inches across, you will be able to see the
dark spots very distinctly and to observe that they are shaded, having a
deep spot in the centre with a paler shadow round it.
[Illustration: Fig. 45.
Face of the sun projected on a sheet of cardboard C. T, Telescope. _f_,
Finder. _og_, Object-glass. _ep_, Eye-piece. S, Screen shutting off the
diffused light from the window.]
As, however, you cannot all use the telescopes, and those who can will
find it difficult to point them truly on to the sun, we will adopt still
another plan. I will turn the object-glass of my portable telescope full
upon the sun's face, and bringing a large piece of cardboard on an easel
near to the other end, draw it slowly backward till the eye-piece forms
a clear sharp image upon it (see Fig. 45). This you can all see
clearly, especially as I have passed the eye-piece of the telescope
through a large screen _s_, which shuts off the light from the window.
You have now an exact image of the face of the sun and the few dark
spots which are upon it, and we have brought, as it were, into our room
that great globe of light and heat which sustains all the life and
vigour upon our earth.
This small image can, however, tell us very little. Let us next see what
photography can show us. The diagram (Fig. 46) shows a photograph of the
sun taken by Mr. Selwyn in October 1860. Let me describe how this is
done. You will remember that there is a point in the telescope tube
where the rays of light form a real image of the object at which the
telescope is pointed (see p. 44). Now an astronomer who wishes to take a
photograph of the sun takes away the eye-piece of his telescope and puts
a photographic plate in the tube exactly at the place where this real
image is formed. He takes care to blacken the frame of the plate and
shuts up this end of the telescope and the plate in a completely dark
box, so that no diffused light from outside can reach it. Then he turns
his telescope upon the sun that it may print its image.
But the sun's light is so strong that even in a second of time it would
print a great deal too much, and all would be black and confused. To
prevent this he has a strip of metal which slides across the tube of the
telescope in front of the plate, and in the upper part of this strip a
very fine slit is cut. Before he begins, he draws the metal up so that
the slit is outside the tube and the solid portion within, and he
fastens it in this position by a thread drawn through and tied to a bar
outside. Then he turns his telescope on the sun, and as soon as he
wishes to take the photograph he cuts the thread. The metal slides
across the tube with a flash, the slit passing across it and out again
below in the hundredth part of a second, and in that time the sun has
printed through the slit the picture before you.
[Illustration: Fig. 46.
Photograph of the face of the sun, taken by Mr. Selwyn, October 1860,
showing spots, faculae, and mottled surface.]
In it you will observe at least two things not visible on our
card-image. The spots, though in a different position from where we see
them to-day, look much the same, but round them we see also some bright
streaks called _faculae_, or torches, which often appear in any region
where a spot is forming, while the whole face of the sun appears mottled
with bright and darker spaces intermixed. Those of you who have the
telescopes can see this mottling quite distinctly through them if you
look at the sun. The bright points have been called by many names, and
are now generally known as "light granules," as good a name, perhaps, as
any other.
This is all our photograph can tell us, but the round disc there shown,
which is called the _photosphere_, or light-giving sphere, is by no
means the whole of the sun, though it is all we see daily with the naked
eye. Whenever a total eclipse of the sun takes place--by the dark body
of the moon coming between us and it, so as to shut out the whole of
this disc--a brilliant white halo, called the crown or _corona_, is seen
to extend for many thousands of miles all round the darkened globe. It
varies very much in shape, sometimes forming a kind of irregular square,
sometimes a circle with off-shoots, as in Fig. 47, which shows what
Major Tennant saw in India during the total eclipse of August 18, 1868,
and at other times it shoots out in long pearly white jets and sheets of
light with dark spaces between. On the whole it varies periodically. At
the time of few sun-spots its extensions are equatorial; but when the
sun's face is much covered with spots, they are diagonal, stretching
away from the spot-zones, but not nearly so far.
[Illustration: Fig. 47.
Total eclipse of the sun, as drawn by Major Tennant at Guntoor in India,
August 18, 1868, showing corona and the protuberances seen at the
beginning of totality.]
And besides this corona there are seen very curious flaming projections
on the edge of the sun, which begin to appear as soon as the moon covers
the bright disc. In our diagram (Fig. 47) you see them on the left side
where the moon is just creeping over the limits of the photosphere and
shutting out the strong light of the sun as the eclipse becomes total. A
very little later they are better seen on the other side just before the
bright edge of the sun is uncovered as the moon passes on its way. These
projections in the real sun are of a bright red colour, and they take on
all manners of strange shapes, sometimes looking like ranges of fiery
hills, sometimes like gigantic spikes and scimitars, sometimes even like
branching fiery trees. They were called _prominences_ before their
nature was well understood, and will probably always keep that name. It
would be far better, however, if some other name such as "glowing
clouds" or "red jets" could be used, for there is now no doubt that they
are jets of gases, chiefly hydrogen, constantly playing over the face of
the sun, though only seen when his brighter light is quenched. They have
been found to shoot up 20,000, 80,000, and even as much as 350,000 miles
beyond the edge of the shining disc; and this last means that the flames
were so gigantic that if they had started from our earth they would have
reached beyond the moon. We shall see presently that astronomers are now
able by the help of the spectroscope to see the prominences even when
there is no eclipse, and we know them to be permanent parts of the
bright globe.
This gives us at last the whole of the sun, so far as we know. There is,
indeed, a strange faint zodiacal light, a kind of pearly glow seen after
sunset or before sunrise extending far beyond the region of the corona;
but we understand so little about this that we cannot be sure that it
actually belongs to the sun.
And now how shall I best give you an idea of what little we do know
about this great surging monster of light and heat which shines down
upon us? You must give me all your attention, for I want to make the
facts quite clear, that you may take a firm hold upon them.
Our first step is to question the sunlight which comes to us; and this
we do with the spectroscope. Let me remind you how we read the story of
light through this instrument. Taking in a narrow beam of light through
a fine slit, we pass the beam through a lens to make the rays parallel,
and then throw it upon a prism or row of prisms, so that each set of
waves of coloured light coming through the slit is bent on its own road
and makes an upright image of the slit on any screen or telescope put to
receive it (see Fig. 21, p. 52). Now when the light we examine comes
from a glowing solid, like white-hot iron, or a glowing liquid, or a gas
under such enormous pressure that it behaves like a liquid, then the
images of the slit always overlap each other, so that we see a
continuous unbroken band of colour. However much you spread out the
light you can never break up or separate the spectrum in any part.[1]
But when you send the light, of a glowing gas such as hydrogen through
the spectroscope, or of a substance melted into gas or vapour, such as
sodium or iron vaporised by great heat, then it is a different story.
Such gases give only a certain number of bright lines quite separate
from each other on the dark background, and each kind of gas gives its
own peculiar lines; so that even when several are glowing together there
is no confusion, but when you look at them through the spectroscope you
can detect the presence of each gas by its own lines in the spectrum.
[1] Two rare earths, Erbia and Didymium, form an exception to this,
but they do not concern us here.
[Illustration: TABLE OF SPECTRA. Plate I.]
To make quite sure of this we will close the shutters and put a pinch of
salt in a spirit-flame. Salt is chloride of sodium, and in the flame the
sodium glows with a bright yellow light. Look at this light through your
small direct-vision spectroscopes[1] and you see at once the bright
yellow double-line of sodium (No. 3, Plate I.) start into view across
the faint continuous spectrum given by the spirit-flame. Next I will
show you glowing hydrogen. I have here a glass tube containing hydrogen,
so arranged that by connecting two wires fastened to it with the
induction coil of our electric battery it will soon glow with a bright
red colour. Look at this through your spectroscopes and you will see
three bright lines, one red, one greenish blue, and one indigo blue,
standing out on the dark background (No. 4, Plate I.)
[1] A direct-vision spectroscope is like a small telescope with
prisms arranged inside the tube. The object-glass end is covered by
two pieces of metal, which slide backwards and forwards by means of
a screw, so that a narrow or broad slit can be opened.
Think for a moment what a grand power this gives you of reading as in a
book the different gases which are glowing in the sky even billions of
miles away. You would never mistake the lines of hydrogen for the line
of sodium, but when looking at a nebula or any mass of glowing gas you
could say at once "sodium is glowing there," or "that cloud must be
composed of hydrogen."
Now, opening the shutters, look at the sunlight through your
spectroscopes. Here you have something different from either the
continuous spectrum of solids, or the bright separate lines of gases,
for while you have a bright-coloured band you have also some dark lines
crossing it (No. 2, Plate I.) It is those dark lines which enable us to
guess what is going on in the sun before the light comes to us. In 1859
Professor Kirchhoff made an experiment which explained those dark lines,
and we will repeat it now. Take a good look at the sunlight spectrum, to
fix the lines in your memory, and then close the shutters again.
[Illustration: Fig. 48.
Kirchhoff's experiment, explaining the dark lines in sunlight.
A, Limelight dispersed through a prism. _s_, Slit through which the beam
of light comes. _l_, Lens bringing it to a focus on the prism _p_. _sp_,
Continuous spectrum thrown on the wall. B, The same light, with the
flame _f_ containing glowing sodium placed in front of it. D, Dark
sodium line appearing in the spectrum.]
I have here our magic-lantern with its lime-light, in which the solid
lime glows with a white heat, in consequence of the jets of oxygen and
hydrogen burning round it. This was the light Kirchhoff used, and you
know it will give a continuous bright band in the spectroscope. I put a
cap with a narrow slit in it over the lantern tube, so as to get a
narrow beam of light; in front of this I put a lens _l_, and in front of
this again the prism _p_. The slit and the prism act exactly like your
spectroscopes, and you can all see the continuous spectrum on the screen
(_sp_, A, Fig. 48). Next I put a lighted lamp of very weak spirit in
front of the slit, and find that it makes no difference, for whatever
light it gives only strengthens the spectrum. But now notice carefully.
I am going to put a little salt into the flame, and you would expect
that the sodium in it, when turned to glowing vapour, causing it to look
yellow, would strengthen the yellow part of the spectrum and give a
bright line. This is what Kirchhoff expected, but to his intense
surprise he saw as you do now a _dark line_ D start out where the bright
line should have been.
What can have happened? It is this. The oxyhydrogen light is very hot
indeed, the spirit flame with the sodium is comparatively weak and cool.
So when those special coloured waves of the oxyhydrogen light which
agree with those of the sodium light reached the flame, they spent all
their energy in heating up those waves to their own temperature, and
while all the other coloured rays travelled on and reached the screen,
these waves were stopped or _absorbed_ on the way, and consequently
there was a blank, black space in the spectrum where they should have
been. If I could put a hydrogen flame cooler than the original light in
the road, then there would be three dark lines where the bright hydrogen
lines should be, and so with every other gas. _The cool vapour in front
of the hot light cuts off from the white ray exactly those waves which
it gives out itself when burning._
Thus each black line of the sun-spectrum (No. 2, Plate I.), tells us
that some particular ray of sunlight has been absorbed by a cooler
vapour _of its own kind_ somewhere between the sun and us, and it must
be in the sun itself, for when we examine other stars we often find dark
lines in their spectrum different from those in the sun, and this shows
that the missing rays must have been stopped close at home, for if they
were stopped in our atmosphere they would all be alike.
There are, by the bye, some lines which we know are caused by our
atmosphere, especially when it is full of invisible water vapour, and
these we easily detect, because they show more distinctly when the sun
is low and shines through a thicker layer of air than when he is high up
and shines through less.
But to return to the sun. In your small spectroscopes you see very few
dark lines, but in larger and more perfect ones they can be counted by
thousands, and can be compared with the bright lines of glowing gases
burnt here on earth. In the spectrum of glowing iron vapour 460 lines
are found to agree with dark lines in the sun-spectrum, and other gases
have nearly as many. Still, though thousands of lines can now be
explained, by matching them with the bright lines of known gases, the
whole secret of sunlight is not yet solved, for the larger number of
lines still remain a riddle to be read.
We see then that the spectroscope teaches us that the round light-giving
disc or photosphere of the sun consists of a bright and intensely hot
light shining behind a layer of cooler though still very hot vapours,
which form a kind of shell of luminous clouds around it, and in this
shell, or _reversing layer_--as it is often called, because it turns
light to darkness--we have proved that iron, lead, copper, zinc,
aluminum, magnesium, potassium, sodium, carbon, hydrogen, and many other
substances common to our earth, exist in a state of vapour for a depth
of perhaps 1000 miles.
You will easily understand that when the spectroscope had told so much,
astronomers were eager to learn what it would reveal about the
prominences or red jets seen during eclipses, and they got an answer in
India during that same eclipse of August 1868 which is shown in our
diagram (Fig. 47). Making use of the time during which the prominences
were seen, they turned the telescope upon them with a spectroscope
attached to it, and saw a number of bright lines start out, of which the
chief were the three bright lines of hydrogen, showing that these
curious appearances are really flames of glowing gas.
In the same year Professor Jannsen and Mr. Lockyer succeeded in seeing
the bright lines of the prominences in full sunlight. This was done in a
very simple way, when once it was discovered to be possible, and though
my apparatus (Fig. 49) is very primitive compared with some now made, it
will serve to explain the method.
[Illustration: Fig. 49.
The spectroscope attached to the telescope for the examination of the
sun. (Lockyer.)
P, Pillar of Telescope. T, Telescope. S, Finder or small telescope for
pointing the telescope in position. _a_, _a_, _b_, Supports fastening
the spectroscope to the telescope. _d_, Collimator or tube carrying the
slit at the end nearest the telescope, and a lens at the other end to
render the rays parallel. _c_, Plate on which the prisms are fixed. _e_,
Small telescope through which the observer examines the spectrum after
the ray has been dispersed in the prisms. _h_, Micrometer for measuring
the relative distance of the lines.]
When an astronomer wishes to examine the spectrum of any special part of
the sun, he takes off the eye-piece of his telescope and screws the
spectroscope upon the draw-tube. The spectroscope is made exactly like
the large one for ordinary work. The tube _d_ (Fig. 49) carries the slit
at the end nearest the telescope, and this slit must be so placed as to
stand precisely at the principal focus of the lens where the sun's image
is formed (see _i_, _i_, p. 44). This comes to exactly the same thing as
if we could put the slit close against the face of the sun, so as to
show only the small strip which it covers, and by moving it to one part
or another of the image we can see any point that we wish and no other.
The light then passes through the tube _d_ into the round of prisms
standing on the tray _c_, and the observer looking through the small
telescope _e_ sees the spectrum as it emerges from the last prism. In
this way astronomers can examine the spectrum of a spot, or part of a
spot, or of a bright streak, or any other mark on the sun's face.
Now in looking at the prominences we have seen that the difficulty is
caused by the sunlight, between us and them, overpowering the bright
lines of the gas, nor could we overcome this if it were not for a
difference which exists between the two kinds of light. The more you
disperse or spread out the continuous sun-spectrum the fainter it
becomes, but in spreading out the bright lines of the gas you only send
them farther and farther apart; they themselves remain almost as bright
as ever. So, when the telescope forms an image of the red flame in front
of the slit, though the glowing gas and the sunlight both send rays into
the spectroscope, you have only to use enough prisms and arrange them in
such a way that the sunlight is dispersed into a very long faint
spectrum, and then the bright lines of the flames will stand out bright
and clear. Of course only a small part of the long spectrum can be seen
at once, and the lines must be studied separately. On the other hand, if
you want to compare the strong light of the sun with the bright lines of
the prominences, you place the slit just at the edge of the sun's image
in the telescope, so that half the slit is on the sun's face and half on
the prominence. The prisms then disperse the sunlight between you and
the prominences, while they only lessen the strong light of the sun
itself, which still shows clearly. In this way the two spectra are seen
side by side and the dark and bright lines can be compared accurately
together (see Fig. 50).
[Illustration: Fig. 50.
Bright lines of prominences.
Sun-spectrum with dark lines.]
Wherever the telescope is turned all round the sun the lines of luminous
gas are seen, showing that they form a complete layer outside the
photosphere, or light-giving mass, of the sun. This layer of luminous
gases is called the _chromosphere_, or coloured sphere. It lies between
the photosphere and the corona, and is supposed to be at least 5000
miles deep, while, as we have seen, the flames shoot up from it to
fabulous heights.
The quiet red flames are found to be composed of hydrogen and another
new metal called helium; but lower down, near the sun's edge, other
bright lines are seen, showing that sodium, magnesium, and other metals
are there, and when violent eruptions occur these often surge up and
mingle with the purer gas above. At other times the eruptions below
fling the red flames aloft with marvellous force, as when Professor
Young saw a long low-lying cloud of hydrogen, 100,000 miles long, blown
into shreds and flung up to a height of 200,000 miles, when the
fragments streamed away and vanished in two hours. Yet all these violent
commotions and storms are unseen by us on earth unless we look through
our magic glasses.
You will wonder no doubt how the spectroscope can show the height and
the shape of the flames. I will explain to you, and I hope to show them
you one day. You must remember that the telescope makes a small real
image of the flame at its focus, just as in one of our earlier
experiments you saw the exact image of the candle-flame upside down on
the paper (see p. 33). The reason why we only see a strip of the flame
in the spectroscope is because the slit is so narrow. But when once the
sunlight was dispersed so as no longer to interfere, Dr. Huggins found
that it is possible to open the slit wide enough to take in the image of
the whole flame, and then, by turning the spectroscope so as to bring
one of the bright hydrogen lines into view, the actual shape of the
prominence is seen, only it will look a different colour, either red,
greenish-blue, or indigo-blue, according to the line chosen. As the
image of the whole sun and its appendages in the telescope is so very
small, you will understand that even a very narrow slit will really take
in a very large prominence several thousand miles in length. Fig 51
shows a drawing by Mr. Lockyer of a group of flames he observed very
soon after Dr. Huggins suggested the open slit, and these shapes did not
last long, for in another picture he drew ten minutes later their
appearance had already changed.
[Illustration: Fig. 51.
Red prominences, as drawn by Mr. Lockyer during the total eclipse of
March 14, 1869.]
These then are some of the facts revealed to us by our magic glasses. I
scarcely expect you to remember all the details I have given you, but
you will at least understand now how astronomers actually penetrate into
the secrets of the sun by bringing its image into their observatory, as
we brought it to-day on the card-board, and then making it tell its own
tale through the prisms of the spectroscope; and you will retain some
idea of the central light of the sun with its surrounding atmosphere of
cooler gases and its layer of luminous lambent gases playing round it
beyond.
Of the corona I cannot tell you much, except that it is far more subtle
than anything we have spoken of yet; that it is always strongest when
the sun is most spotted; that it is partly made up of self-luminous
gases whose bright lines we can see, especially an unknown green ray;
while it also shines partly by reflected light from the sun, for we can
trace in it faint dark lines; lastly it fades away into the mysterious
zodiacal light, and so the sun ends in mystery at its outer fringe as it
began at its centre.
And now at last, having learnt something of the material of the sun, we
can come back to the spots and ask what is known about them. As I have
said, they are not always the same on the sun's face. On the contrary,
they vary very much both in number and size. In some years the sun's
face is quite free from them, at others there are so many that they form
two wide belts on each side of the sun's equator, with a clear space of
about six degrees between. No spots ever appear near the poles. Herr
Schwabe, who watched the sun's face patiently for more than thirty
years, has shown that it is most spotted about every eleven years, then
the spots disappear very quickly and reappear slowly till the full-spot
time comes round again.
Some spots remain a very short time and then break up and disappear, but
others last for days, weeks, and even months, and when we watch these,
we find that a spot appears to travel slowly across the face of the sun
from east to west and then round the western edge so that it disappears.
It is when it reaches the edge that we can convince ourselves that the
spot is really part of the sun, for there is no space to be seen between
them, the edge and the spot are one, as the last trace of the dark
blotch passes out of sight. In fact, it is not the spot which has
crossed the sun's face, but the sun itself which has turned, like our
earth, upon its axis, carrying the spot round with it. As some spots
remain long enough to reappear, after about twelve or thirteen days, on
the opposite edge, and even pass round two or three times, astronomers
can reckon that the sun takes about twenty-five days and five hours in
performing one revolution. You will wonder why I say only _about_
twenty-five, but I do so because all spots do not come round in exactly
the same time, those farthest from the equator lag rather more than a
day behind those nearer to it, and this is explained by the layer of
gases in which they are formed, drifting back in higher latitudes as the
sun turns.
It is by watching a spot as it travels across the sun, that we are able
to observe that the centre partlies deeper in the sun's face than the
outer rim. There are many ways of testing this, and you can try one
yourselves with a telescope if you watch day after day. I will explain
it by a simple experiment. I have here a round lump of stiff dough, in
which I have made a small hollow and blackened the bottom with a drop of
ink. As I turn this round, so that the hollow facing you moves from
right to left, you will see that after it passes the middle of the face,
the hole appears narrower and narrower till it disappears, and if you
observe carefully you will note that the dark centre is the first thing
you lose sight of, while the edges of the cup are still seen, till just
before the spot disappears altogether. But now I will stick a wafer on,
and a pea half into, the dough, marking the centre of each with ink.
Then I turn the ball again. This time you lose sight of the foremost
edge first, and the dark centre is seen almost to the last moment. This
shows that if the spots were either flat marks, or hillocks, on the
sun's face, the dark centre would remain to the last, but as a fact it
disappears before the rim. Father Secchi has tried to measure the depth
of a spot-cavity, and thinks they vary from 1000 to 3000 miles deep. But
there are many difficulties in interpreting the effects of light and
shadow at such an enormous distance, and some astronomers still doubt
whether spots are really depressions.
For many centuries now the spots have been watched forming and
dispersing, and this is roughly speaking what is seen to happen. When
the sun is fairly clear and there are few spots, these genera
Showing first 200,000 characters. Full text: gutenberg.org/ebooks/37589