Motion Picture Operation, Stage Electrics and Illusions
by Henry Charles Horstmann · English · Gutenberg #45231 · 565,625 chars · public domain (US)
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Title: Motion Picture Operation, Stage Electrics and Illusions
Author: Henry Charles Horstmann
Victor H. Tousley
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Language: English
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Books by
Horstmann & Tousley
Motion Picture Operation, Stage Electrics and Illusions. $2.00
(Ready July 1, 1914.)
Alternating Current Theory, Practice and Diagrams. $2.00
Modern Electric Illumination. $2.00
Practical Armature and Magnet Winding. $1.50
Modern Electrical Construction. $1.50
Electrical Wiring and Construction Tables. $1.50
Dynamo Tending for Engineers. $1.50
Modern Wiring Diagrams and Descriptions. $1.50
Electricians’ Operating and Testing Manual. $1.50
Motion Picture Operation
STAGE ELECTRICS
AND ILLUSIONS
A Practical Hand-book and Guide for Theater
Electricians, Motion Picture Operators and
Managers of Theaters and
Productions
By
Henry C. Horstmann
and
Victor H. Tousley
Authors of
“Alternating Current,” “Modern Wiring Diagrams,” “Modern
Electrical Construction,” “Electrical Wiring and Construction
Tables,” “Practical Armature and Magnet
Winding,” “Electricians’ Operating and Testing
Manual,” “Modern Illumination.”
ILLUSTRATED
[Illustration]
Chicago
FREDERICK J. DRAKE & CO.
Publishers
Copyright 1914, by
HENRY C. HORSTMANN
and
VICTOR H. TOUSLEY
PREFACE
In this volume the authors have attempted to lay before the
Motion-Picture Operators and Theatrical Employes generally, a reference
and handbook making a specialty of electrical requirements about
theaters.
A working knowledge of electricity in general is assumed, and therefore
elementary ideas have been treated sparingly. A specialty, however, has
been made of all matters peculiar to theaters, and it is thought that
theater electricians will find in this volume everything that they need
whether they be operating motion-picture machines or switchboards in
first-class houses.
The two special chapters “Portable Stage Equipment” and “Theater Wiring”
have been arranged so that they are particularly valuable for reference.
They should be consulted before undertaking any electrical construction
work, either for the stage or for the auditorium. These chapters embody
all of the practical knowledge that has come to the notice of the
authors in many years of actual experience with theatrical construction.
The aim of this volume has been to present in a simple and practical way
the essential principles of Motion-Picture Work.
THE AUTHORS.
Table of Contents
PAGE
CHAPTER I
The Electrical Circuit and Electrical Hazards 9
CHAPTER II
The Arc Lamp 19
CHAPTER III
Projection 31
CHAPTER IV
Motion Pictures 55
CHAPTER V
The Motion-Picture Machine 62
CHAPTER VI
The Film 89
CHAPTER VII
General Hints on Installation, Operation and Care of Machines 96
CHAPTER VIII
Light 113
CHAPTER IX
Principles of Vision 122
CHAPTER X
Reflection 126
CHAPTER XI
Refraction 137
CHAPTER XII
Optical Instruments 147
CHAPTER XIII
Optical Illusions 155
CHAPTER XIV
Theater Buildings 163
CHAPTER XV
Operating Room Equipment 176
CHAPTER XVI
Current Control for Arc Lamps 190
CHAPTER XVII
Management of Generators and Motors 213
CHAPTER XVIII
Theater Wiring 218
CHAPTER XIX
Portable Stage Equipment 311
CHAPTER XX
Useful Facts and Formulas 353
CHAPTER XXI
Glossary of Electrical, Mechanical and Optical Words, Terms and
Phrases 358
Index 385
MOTION PICTURE OPERATION
STAGE ELECTRICS AND
ILLUSIONS
CHAPTER I.
THE ELECTRICAL CIRCUIT AND ELECTRICAL HAZARDS.
_Two and Three-Wire Systems._--If the theater electrician will take the
trouble to trace the circuits in the building to their supply, he will
find them entering the building either as two-wire or three-wire
circuits.
[Illustration: FIGURE 1.]
A two-wire circuit is diagrammatically shown in Figure 1. The circuit,
coming from _1_, enters the building, passes through the fuses _2_, and
through switch 3 to the lights. A two-wire system will ordinarily be
found operating at 110 volts, the current varying according to the
number of lights turned on. In the drawing, for instance, only one light
is shown with the switch closed, the other three switches being open.
The current in the circuit is equal to that which passes through the
single lamp. If another switch be closed, another light will burn and
the current will be increased, so that the more lights be turned on, the
greater will be the current.
[Illustration: FIGURE 2.]
The three-wire system, Figure 2, is almost universally used where the
supply is from the outside and where any considerable number of lights
are connected. The chief advantage of the three-wire system lies in its
economy of copper. The middle or neutral wire ordinarily does not carry
current, but it is a necessity whenever the number of lights burning on
the two sides of the system are not equal.
With the neutral wire omitted, we have a straight two-wire system using
double the voltage of the ordinary two-wire system and always operating
two 110-volt lamps in series. Two lamps would always have to be turned
on at the same time and if one of them should burn out, the other would
be extinguished also.
A system using double voltage requires only half the current and
consequently but half the copper. In order to obviate the necessity of
always using two lamps together and at the same time economizing in
copper, the neutral wire is provided. As long as the same number of
lamps are burning on each side of the neutral wire, the same current
always passes through two lamps in series and there is no current in
the neutral. Should, however, the group on one side be turned out, the
other would still continue to burn; but the path of the current to the
dynamo, or bank of transformers, would be through the neutral wire.
The system is thus seen to possess all the advantages of the ordinary
two-wire system since each lamp can be operated by itself and at the low
voltage, while the actual supply voltage for the whole system is double
that which is actually used at any lamp. We have thus two voltages at
our command; 110 and 220 being the voltages in common use.
_Electrical Hazards._--Since this work is not intended for mere
beginners, we shall not enter into elementary considerations, but shall
take up the matter of fire and life hazard, both of which are important
items to which attention cannot be too often called.
The electrical current may cause fire by overheating the wires. This
overheating may be due to the willful overloading of circuits; and to
prevent this, no wire should ever be used to carry more current than is
allowed by the table of carrying capacities given on page 238.
The overheating may also be due to an unknown load which is caused by a
“ground” or a partial short circuit. “Ground” is the technical term used
to designate the connection of a wire to any substance over which
electricity may be carried to the wire of opposite polarity. A ground
may be caused by a bare wire coming in contact with the iron framework
of a building, wet wood, or moist substances of any kind. One such
ground on a circuit can do no harm; but, if one ground exists, in the
course of time another one may come on and, when the second one
appears, if it is on the side of the circuit opposite to the first,
there will be trouble at once.
If the two grounds are both “good”--that is, if they are of low
resistance--we shall have a short circuit and probably blow a fuse; but
if they are not “good,” we may have but a small current which may
continue unnoticed for months. Such a current may eat away the copper of
the positive pole and in time cause the wire to break, creating an
electric arc and perhaps causing a fire. It may also cause wet wood
through which it flows to become charred and finally ignited.
The ground is the bane of the electrical worker. If a system can be kept
free of grounds, the chances of trouble are vastly reduced. The cause of
most grounds is moisture. Nearly all substances except metals are fairly
good insulators if dry; and nearly all of them are fairly good
conductors if sufficiently wet.
Another very prolific source of fires is the electrical spark, large and
small. The spark, due to the breaking of an incandescent lamp, often
causes fires when it comes in contact with inflammable material or
gases. The ordinary lamp cord also causes many fires because it is
easily damaged and liable to short circuits which often result in
arcing. Short circuiting two wires or breaking a wire carrying current
may easily ignite inflammable material in the vicinity.
The best way to reduce the fire hazard to a minimum, is to install all
electrical work carefully according to the rules laid down in the
“National Electrical Code”.
The life hazard is one which concerns the operator personally and is
especially great to those traveling with shows. Traveling men are often
obliged to get along with all sorts of make shifts, especially in the
smaller towns where one-night stands are the rule. Here it is often
necessary to connect to trolley circuits or power lines of different
voltages, frequencies, etc.
A person may suffer injury directly from a current of electricity by
making himself part of the circuit. If the system on which he is working
is alive and grounded, he may easily cause injury to himself by touching
a live wire with his hands while standing upon anything that is
grounded. By so doing he completes a path for the current through his
body.
He may also become part of a circuit by holding a wire with both hands
while someone is cutting it between his two hands. As long as a wire so
held is intact, no shock can be received except to ground, but when the
wire is cut or breaks, a very high voltage may be produced for an
instant which will cause a current through the body of the man holding
the wire. The extra high voltage is produced only if the wire is
carrying current at the time it is being cut. Under these circumstances
there will often be a strong flash, due to the momentary increase in
voltage, produced by the breaking of the circuit, which may be
excessive, especially if there is considerable inductance in the
circuit.
The most frequent cause of injury is due to making contact with the two
opposite polarities of a system. As a rule circuits, with which
operators ordinarily have to work, are low voltage, i.e., not over 220
volts. But many deaths have been caused by this voltage, sometimes
directly and at other times indirectly as, for instance, by causing a
fall. People whose hearts are in any way defective should be careful
about exposing themselves to shocks even at 110 volts.
It is true that many wiremen are in the habit of testing 220-volt
circuits by allowing the current to pass through their bodies, but it
will be noticed that they are very careful not to make a good contact.
The current which passes through the body, when one touches two wires of
opposite polarity very lightly with the finger tips, is but a fraction
of what one would receive if he were unwittingly to grab two wires of
opposite polarities with the hands, especially if the hands were moist.
Numerous cases are on record of persons having been killed by 110 volts
under favorable circumstances; as, for instance, while in the bath
receiving a shock from a so-called vibrator. The body partly immersed in
water and perhaps a foot resting against a water pipe forms a conductor
of very low resistance, and a comparatively strong current may pass
through the body.
The most important precautions against injury while working on live
circuits are:
(1) Insulate yourself from the ground.
(2) Handle only one side of a line at a time.
(3) If possible, work with only one hand at a time in contact with the
wires.
(4) Use rubber gloves, or rubber boots where necessary, but bear in mind
that they are of little value unless kept dry. Moisture will allow some
current to pass over the surface of any substance no matter how good an
insulator it may otherwise be.
(5) Always place yourself so that a slight shock which might cause you
to lose your balance will not give you a bad fall.
(6) Remember that if once you make good contact with an
alternating-current circuit, you cannot let go.
(7) Fix firmly in your mind the directions for resuscitation from
electric shock, on pages 15-18.
When energy is obtained through transformers, there is another danger to
be added to the above, viz., the possibility of the breaking down of the
insulation between primary and secondary wires of the transformer. If
this happens, we have suddenly and without warning, instead of the 110
or 220 volts supposed to exist between the wires forming the circuit,
2,000 or 3,000 volts. Such accidents are especially likely during
thunder storms when lightning often breaks down transformers.
In order to reduce this danger to a minimum, the secondaries of
transformers are grounded. It will be well for the electrician to assure
himself that the secondaries of the transformers from which he is
getting his supply are grounded. This can be tested by an incandescent
lamp. Connect the lamp to ground with one wire and, with the other, try
the two sides of the circuit. If the transformer secondaries are
properly grounded, the lamp will burn at full candle power from one of
the wires; this will show that the other wire is grounded.
A person working on such a circuit is of course more likely to receive a
low voltage shock than if the secondaries were not grounded, but he is
fairly well protected against the primary voltage or lightning.
RESUSCITATION FROM ELECTRIC SHOCK.
Rules recommended by commission on resuscitation from electric shock,
representing The American Medical Association, The National Electric
Light Association, The American Institute of Electrical Engineers: Dr.
W. B. Cannon, chairman; professor of physiology, Harvard University. Dr.
Yandell Henderson, professor of physiology, Yale University; Dr. S. J.
Meltzer, head of department of physiology and pharmacology, Rockefeller
Institute for Medical Research; Dr. Edw. Anthony Spitzka, director and
professor of general anatomy, Daniel Baugh Institute of Anatomy,
Jefferson Medical College; Dr. George W. Crile, professor of surgery,
Western Reserve University; W. C. L. Eglin, past-president National
Electric Light Association; Dr. A. E. Kennelly, professor of electrical
engineering, Harvard University; Dr. Elihu Thomson, electrician, General
Electric Company; W. D. Weaver, secretary, editor Electrical World.
Issued and copyrighted by National Electric Light Association. Reprinted
by permission. Follow these instructions even if victim appears dead.
I. IMMEDIATELY BREAK THE CIRCUIT.
With a single quick motion, free the victim from the current. Use any
_dry non-conductor_ (clothing, rope, board) to move either the victim or
the wire. Beware of using metal or any moist material. While freeing the
victim from the live conductor have every effort also made to shut off
the current quickly.
II. INSTANTLY ATTEND TO THE VICTIM’S BREATHING.
1. As soon as the victim is clear of the conductor, rapidly feel with
your finger in his mouth and throat and remove any foreign body
(tobacco, false teeth, etc.) Then _begin artificial respiration at
once_. Do not stop to loosen the victim’s clothing now; _every moment of
delay is serious_. Proceed as follows:
_a._ Lay the subject on his belly, with arms extended as straightforward
as possible and with face to one side, so that nose and mouth are free
for breathing, see Figure on page 17. Let an assistant draw forward the
subject’s tongue.
[Illustration: INSPIRATION--PRESSURE OFF.]
_b._ Kneel straddling the subject’s thighs and facing his head; rest the
palms of your hands on the loins (on the muscles of the small of the
back), with fingers spread over the lowest ribs, as in Figure on page
17.
_c._ With arms held straight, swing forward slowly so that the weight of
your body is gradually, but _not violently_, brought to bear upon the
subject, see Figure on page 18. This act should take from two to three
seconds.
Immediately swing backward so as to remove the pressure, thus returning
to the position shown in the Figure on page 17.
_d._ Repeat deliberately twelve to fifteen times a minute the swinging
forward and back--a complete respiration in four or five seconds.
_e._ As soon as this artificial respiration has been started, and while
it is being continued, an assistant should loosen any tight clothing
about the subject’s neck, chest or waist.
[Illustration: EXPIRATION--PRESSURE ON.]
2. Continue the artificial respiration (if necessary, at least an hour),
_without interruption_, until natural breathing is restored, or until a
physician arrives. If natural breathing stops after being restored, use
artificial respiration again.
3. _Do not give any liquid by mouth until the subject is fully
conscious._
4. Give the subject fresh air, but keep him warm.
III. SEND FOR NEAREST DOCTOR AS SOON AS ACCIDENT IS DISCOVERED.
CHAPTER II.
THE ARC LAMP.
_General Discussion of the Electrical Arc._--The name of the electrical
arc lamp is derived from the arch-like appearance of the vapors which
give out the light when the carbons are placed horizontally. The
horizontal arc was the earliest form, hence the name which it carries to
this day.
The arc proper is due to the vapors of volatilized carbon or other
materials forming the electrodes, which may be consumed by the passage
of an electrical current from one electrode to another through the
intervening medium. In order that an arc may be formed, it is necessary
first to bring the electrodes together. This, if the circuit is properly
arranged, starts the current and when the circuit is partly interrupted,
as by slowly separating the points of the electrodes, the current passes
through the intervening space, with the result that a high degree of
heat (about 3,500 centigrade) is produced. This results in volatilizing
the carbon or any other material of which one or both electrodes may
consist.
As long as the distance between the electrode points is small, the
current will be quite strong and a hissing or frying sound will be given
out. In order to keep the current within bounds during the time that the
electrodes are together or while they are separated only a very
short distance, some resistance, or reactance in the case of
alternating-current arcs, is always connected in series in the circuit.
If this were not done, there would be a short circuit at the time of
starting or striking the arc.
The arc formed with very short separation of electrodes is generally
spoken of as a low tension arc and requires very hard carbons and about
25 volts. This type of arc is very little used for illuminating
purposes.
If the distance between the electrodes is increased gradually, the light
becomes very unsteady and flickers considerably until at a certain point
it begins to improve and give the long quiet arc. This condition will
occur when, with direct current, the electrodes are about one-eighth of
an inch apart. It will then be found that the voltage across the arc is
from 45 to 50 volts, which is the best voltage to use with open arcs. If
the separation be carried still further, the arc will grow longer and
become flaming until finally it breaks entirely.
The resistance of the arc is closely proportional to the cross section
of the electrodes and increases with the distance of the arc gap. It
acts, however, very much as though there were a small counter e. m. f.
set up within it.
The color of the light given off varies with the length of the arc
somewhat, but depends mainly upon the material of which the electrodes
consist. In the so-called flaming arcs, the peculiar color is obtained
by certain chemicals imbedded in the material composing the electrodes.
Whenever an arc is allowed to burn down until it reaches the electrode
holders, a greenish light is given off which is due to the
volatilization of the metal--usually brass--in these holders.
The light of a strong arc is extremely injurious to the eyes and should
only be viewed through colored glass. Many very painful experiences have
resulted from persons gazing upon arcs of 200 or 300 amperes, such as
are used sometimes in cutting away metals of old buildings, etc.
The most powerful arcs known at the present time are those used in some
steel mills for refining steel. These use upward of 10,000 amperes.
[Illustration: FIGURE 3.]
[Illustration: FIGURE 4.]
The length of the ordinary arc varies from one thirty-second of an inch
to one inch. The light is not of much use and is rather unsteady until
the electrodes have assumed a shape somewhat similar to that shown in
Figure 3 for direct current, and Figure 4 for alternating current. With
direct-current arcs, a crater is formed at the bottom of the positive
electrode and, from this crater, about 80 per cent of the light is
emitted. Where the light is wanted in a downward direction, the crater
is always formed at the top and for this purpose the top electrode must
be made positive; that is, the electricity must flow from the top
electrode into the lower one. In some cases, where special illumination
effects are desired, the bottom electrode is made the positive with the
result that most of the light is thrown upward. In such cases strong
shadows are thrown against the ceiling and the lamp is said to be
burning “upside down.”
The positive electrode can always be distinguished from the negative (a)
by the shadows cast; (b) by the form of the electrodes; and (c) by the
fact that since it is heated to a greater degree, it will, when the lamp
is turned off, remain hot for some time after the negative electrode has
cooled off.
In case the arc is drawn out very long and operated in this way for a
considerable time, the crater will almost wholly disappear and the
electrodes will appear rounded off.
In an alternating-current circuit, the positive and negative poles
reverse generally about 120 times per second and both electrodes in the
alternating-current arc are positive and negative to the same degree.
They are therefore very nearly alike, except that the heat rising from
the lower one increases slightly the volatilization of the upper. The
positive electrode in the direct-current arc is consumed approximately
twice as fast as the negative electrode. The consumption of the two
electrodes in an alternating-current arc is about equal and a crater
much smaller than the kind formed in a direct-current arc is, therefore,
formed on each electrode, instead of only on the positive electrode as
in the case of the direct-current arc.
The general form of alternating-current arc carbons is given in Figure
4. The small elevations shown in the cuts are due to impurities and do
not appear with first-class carbons.
When arc lamps are operated on alternating-current circuits, the best
voltage for the arc is about 28; and consequently, for the same quantity
of light, the current must be increased so that the amperage of
alternating-current lamps is always much greater than that of
direct-current lamps.
The alternating-current arc is much noisier than the direct-current arc,
but with very high frequencies this noise ceases.
In general, arc lamps do not work very well on low frequencies. The time
at which the current is practically zero is long enough to allow the
vapor between the electrode points to cool off sufficiently to interfere
with successful operation.
Any arc light is affected by draughts of air and can even be blown out.
If this occurs often, there will be rapid feeding, a short arc, and
great waste of electrode.
A magnet held close to an arc can be made to blow it out or force it to
one side. This fact is made use of in some lightning arresters.
Generally speaking, arc lamps are of two kinds, _open_ and _enclosed_.
The enclosed arc operates at a much higher voltage and is but little
used about theaters. The open arc is almost universally used for stage
work and this is about the only place where it is still considered
useful. This kind of arc lamp is, however, very hazardous in localities
where inflammable material abounds and for this reason it is always
enclosed with wire mesh when possible.
Lens lamps can be tightly enclosed since none of the light is wanted
except that which passes through the lens in front.
[Illustration: FIGURE 5.]
[Illustration: FIGURE 6.]
The so-called flood lamps are usually provided with wire gauze in front
of the arc, which prevents the escape of pieces of the electrodes and
also prevents parts of scenery, etc., from coming in contact with the
arc.
The lamp houses should be of such dimensions that, with the highest
amperage the lamp is capable of using, the outer walls will not become
excessively hot.
Illustrations of standard lens and flood lamps, as made by the Chicago
Stage Lighting Company, are shown in Figures 5 and 6.
_Operation of Arc Lamps._--From the standpoint of operation, arc lamps
may be divided into two classes, viz.: _hand-feed_ and _automatic-feed_.
The hand-feed lamp is generally used in theaters and is practically the
only kind admitted on the stage, or for stage illuminating purposes.
Only a very few houses now use arc lamps for general illumination.
The operation of hand-feed lamps[1] is ordinarily quite simple and will
be fully treated under the head of “Projection”, so that we may now
consider only the automatic lamps. At the present time these are used
mostly, if at all, for the illumination of the exterior of the theater.
[1] Full diagrams and descriptions are in another work of the
authors’, entitled “Electricians’ Operating and Testing Manual,” so
that no space need be given to these here, save in a very general way.
The operator should first familiarize himself with the construction and
principles upon which the mechanism of his lamp is based. For this
purpose he should remove the outer jacket, thus exposing the working
mechanism; turn on the current; and endeavor to learn the significance
of each part. It is of course necessary that the operator understand the
hazards due to manipulating live wires and that he should be very
careful not to make short circuits or grounds which might destroy parts
of the lamp.
Automatic-feed lamps are usually trimmed in the following manner: Bring
the lamp within reach; remove the globe; take out the lower electrode;
let down the upper electrode rod and thoroughly clean it with crocus
cloth. This upper electrode rod is the principal thing that concerns the
lamp trimmer; it must be perfectly straight and care must be exercised
not to bend it accidentally; it must be clean so that the clutches may
properly grip it; it must not be greasy. If it grows dirty or greasy, it
will soon become pitted from the current that passes from the contacts
to it.
The next operation is to remove the upper electrode and place it in the
lower electrode holder. (The length of electrode necessary should be
known. The lower one generally burns out first--it being shorter--and if
the arc reaches the lower electrode holder, will begin to consume it; if
the lower carbon is too long, the arc is liable to reach the upper
electrode holder and destroy it.) The upper electrode may then be placed
in position and aligned with the lower. To do this it is best to turn it
about and try it until it aligns in all positions. The two electrodes
should form a straight line, up and down, no matter which way the upper
is turned.
In some forms of enclosed lamps, the clutch grips the electrode direct.
In such a case all of the upper electrode must be carefully examined to
see that it is straight and free from burs, and that it can pass freely
into the opening at the top of the inner globe. The successful operation
of enclosed arcs depends upon the confinement of the gas in the inner
globe. This globe must, therefore, be kept as tight as possible without
interfering with the operation of the electrodes which pass through it.
With enclosed arcs, the care of the inner globe is of great importance,
because impurities are cast off which soon coat the inner globe and
absorb much of the light.
The care of the outer globes in general is also an important matter. A
dirty globe looks very unsightly and absorbs much light.
The following points should be carefully considered in handling and
trimming lamps:
(1) Be sure that you understand your system and know whether it is a
constant-current or a constant-potential system of distribution. With
constant-current systems, the current is constant and the voltage over
the arc is regulated; while with constant-potential systems, the voltage
is constant and the current through the arc is regulated.
(2) With constant-current or series lamps, the line must never be
opened, but must be shunted around the lamp if a lamp is to be cut out.
(3) With constant-potential lamps, the lamp must never be shunted but
the circuit must be opened.
(4) In all cases each lamp should be controlled by a double pole switch.
(5) Constant-potential lamps cannot be operated without resistance in
the circuit; this resistance may be in the lamp itself or outside.
(6) Never handle high tension lamps without insulating yourself from the
ground; and handle live wires only with one hand at a time.
(7) Provide spark arresters for all open-arc lamps in the vicinity of
inflammable material.
(8) Never leave a lamp without globes where the wind can strike it. It
will be blown out or feed often, thus consuming the electrodes very fast
and at the same time yielding a very poor light.
Green light emitted by the lamp will indicate that the electrode
holders are burning. Strong shadows cast upwards indicate a lamp burning
“upside down”. The positive electrode retains heat longer than the
negative. The quality and size of electrodes has much to do with
successful operation. Always use the kind of electrodes recommended by
the maker of the lamp.
Direct-current arc lamps do not require much in the shape of reflectors
as they naturally throw most of the light downward, when the upper
electrode is positive. They should as a rule be suspended high.
Alternating-current arc lamps throw most of the light from the upper
electrode slightly below the horizontal and that from the lower
electrode somewhat above. If the light is wanted in a downward
direction, suitable reflectors must be provided.
_Testing of Arc Lamps._--The constant-potential arc lamp is usually
designed for a certain current and voltage. The enclosed arcs as a rule
operate singly on 110 volts, while open arcs are run two in series on
the same voltage. In order to test and see that the voltage and current
are right, an ammeter and a voltmeter are needed. The current and
voltage can both be adjusted by altering the resistance, which is always
in series with such lamps. To get the correct voltage over the arc, be
sure to connect the voltmeter to the two electrode holders so as to
eliminate any other potential drops that may affect the reading.
_Testing Carbons._--The color of the light and the steadiness of it can
of course only be determined by actual operation tests. The arc obtained
by using large electrodes with low current density is liable to rotate
around the electrodes, burn unsteadily, and flicker. This is due to the
fact that the arc tends to establish itself at the point of least
resistance. In order that the arc may burn uniformly, the current
density must be great enough to force all of the electrode points into
use.
As a rule the best electrode is the one that has the longest range from
the low voltage point of hissing to the high voltage point of flaming.
With such an electrode the greatest range in light can be obtained
without either the hissing or the flaming.
The same qualities that give an electrode long range, as above, also
indicate its purity and if we make a test for range, we shall therefore
at the same time make a test for purity.
The test for range can be carried out by any ordinary hand-feed lamp. To
make it, the electrodes are inserted and allowed to burn until their
points have assumed the proper shape. The arc can then be shortened
until the familiar hissing sound is heard. Note the voltage at which
this occurs, being careful to have the voltmeter connected so as to get
the voltage across the arc only. Now separate the electrodes slowly
until they begin to flame and note this voltage. Ordinarily the hissing
voltage will be about 42 and the flaming voltage about 62. The greater
the difference between the two, the better the carbons are assumed to
be. In making comparative tests on electrodes in this manner, care
should be taken that all of the conditions of current and size of
electrodes be the same.
The test for comparative life of electrodes is best made by arranging
the different electrodes so that the same current will pass through each
for the same length of time. If this is done, all that is necessary is
to weigh the electrodes before and after burning. The approximate useful
life of an electrode can be easily determined by burning it for a stated
length of time, noting the length consumed and comparing it with the
length available for burning.
CHAPTER III.
PROJECTION.
_Setting and Adjustment of Carbons._--To project a picture upon a screen
properly is an art and requires close study and some knowledge of all
the factors involved. The most important factor is that of the light.
Electric light is so universally used at the present time that it is
hardly necessary to mention the other sources of illumination.
[Illustration: FIGURE 7.]
The electric current with which the operator has to deal may be either
alternating or direct, and the kind is of great importance. The color of
the light obtained from a direct-current arc is not only superior to
that obtained from an alternating-current arc but is obtained at a much
lower cost since, as we shall presently see, it is so much more
efficient.
To project clear white light upon the screen is impossible, some color
will always be in it. But by careful attention and by training himself
to notice slight degrees of color, the operator can learn to render a
light which will be clear enough to satisfy the majority of the
spectators. In order to obtain this light, the source from whence it
comes should be located exactly in the optical axis of the lens system;
that is, a straight line drawn through the center of all of the lenses
should pass also directly through the center of the arc as indicated in
Figure 7. (For comprehensive treatise on lenses, see Chapter XII.)
[Illustration: FIGURE 8.]
[Illustration: FIGURE 9.]
[Illustration: FIGURE 10.]
[Illustration: FIGURE 11.]
[Illustration: FIGURE 12.]
[Illustration: FIGURE 13.]
Most of the light, we have already seen, is emitted from the crater of
an arc of which there is but one in a direct-current arc and two in an
alternating current arc. In order to obtain the most light with the
least expenditure of current and heat in the lamp house, the crater must
be formed in such a manner as to face the center of the condensers as
nearly as possible. Since, however, there are always two electrodes and
the current must pass from one to the other, the crater always tends to
face the lower electrode if the upper one is positive. It is, therefore,
impossible to get the full benefit of the light for the condenser; we
must be satisfied with getting a part of it, and to do this such
settings of electrodes as are shown in Figures 8 to 13 are used. About
the relative merits of these various settings there is considerable
dispute and the best advice that can be given to any new comer in the
operating line is to make his own experiments and find out for himself.
The fact that a certain point is much disputed, alone indicates that
there is no exact knowledge available; for we very seldom have any
differences of opinion about the things that we can prove.
In the operating line very much depends upon the judgment of the
operator. Electrode setting like that of Figure 8 may be good for an
operator who is extremely careful and has a reliable machine which
requires a minimum of attention. But it can readily be seen that if the
top electrode were fed a trifle too far forward, the crater would form
underneath and the lenses would receive but a small part of the light.
Each of the settings given has its peculiarities and it is best for any
operator who has not done so, to try them all out and find which one
best suits him and his conditions.
Figures 8 to 10 show the settings used with direct-current arcs; while
those illustrated by Figures 11 to 13 are used with alternating-current
arcs.
With alternating-current arcs the problem is even more difficult than
with direct, for we have here two craters to deal with; and if we wish
to use the light from both, we shall have to be very careful about it.
If the electrodes are not set exactly right, we may get a double spot
and poor illumination at the center of the screen. Perhaps most
operators will soon give up the idea of using the light of both craters
and will settle down to an electrode setting something like that shown
in _A_, Figure 7. In this setting both electrodes are angled and the
lower one is set a little ahead of the upper. This has a tendency to
draw the crater of the upper electrode forward, thus improving the light
on the condenser; but if this be carried too far, the lower electrode
will obstruct the lower part of the lens. The lower electrode must
always be set so that it allows all parts of the condenser to receive
direct rays of light from the crater of the upper. The electrode must
align perfectly in the vertical plane as shown in _B_, Figure 7, or the
arc will move while burning.
In order to enable the operator to arrange his electrodes at any angle
and to bring them into the center of the optical system, arc lamps are
made up in various ways as illustrated in Figures 14 to 19. The simpler
types are used only in stage lighting lamps where the centering is not
so important. The more elaborate lamps are provided for motion picture
arc lamps and allow of all necessary adjustments which are: feed
electrodes; move lamps forward or back; up or down; sideways and angle
electrodes.
Where direct current is used, the upper electrode must be fed
approximately twice as fast as the lower; but with alternating current,
they both feed at practically the same rate.
[Illustration: FIGURE 14.]
Figure 14 shows a form of McIntosh stereopticon lamp.
[Illustration: FIGURE 15.]
Figure 15 is a Kliegl lamp for open arc lamps.
[Illustration: FIGURE 16.]
Figure 16 is an Edison lamp used for motion picture work.
[Illustration: FIGURE 17.]
Figure 17 is a Kliegl lamp used for focusing purposes.
[Illustration: FIGURE 18.]
Figure 18 shows the Powers lamp.
[Illustration: FIGURE 19.]
Figure 19 shows one of the Motiograph Company lamps.
_Optical System._--In Figure 20 we have the complete optical system of
the moving picture or stereopticon outfit. The crater of the arc lamp
and the center of the objective lens are at the conjugate focal points
(see Optics) and must always be in this relation. The size of the
picture projected upon the screen is governed entirely by the focal
length of the objective lens and the distance of the screen from this
lens. The shorter the focal length, the greater will be the bulging out
or rounding of the lens, and the larger the picture projected. The
objective lens is always fitted with an adjusting device of some kind by
which it can be moved forward or back a little to focus the picture
properly.
[Illustration: FIGURE 20.]
In order to project a picture properly, it is necessary that the center
of the arc or other illuminant, the center of the condensers, and the
center of the objective, all fall in one straight line as indicated in
Figure 20. The condensers are provided for the purpose of gathering and
condensing as many of the scattering light rays of the arc lamp as
possible and bringing them to bear upon the slide and the objective.
The light used must come either from a reasonably small source or from a
larger source far enough away so that the rays can be considered as
parallel. The focal point for parallel rays would, however, differ
somewhat from that of a point source and such illumination is seldom
used; in fact, it is used only where special arrangements are made for
it.
One of the principal points to be borne in mind in trying to project a
good clear picture is to keep the arc down to as small a point as is
practicable. A long arc can be tolerated only when it is absolutely
impossible to obtain sufficient illumination from a short arc; as, for
instance, in operating the Kinemacolor machines, in which from 80 to 100
amperes are used with a very long arc. The above expedient is imperative
because the colored discs through which the light must pass absorb a
great amount of it and the definition or outline of the picture is apt
to be poor.
The position of the arc with reference to the condensers is also an
important point to consider. The focal length of the condensers
determines the point at which the arc must be maintained. The flatter
the condensers are, the farther away the arc may be, and the less will
be the heating; but this position is accompanied with considerable loss
of light.
For the purpose of projection we can use only the light which strikes
the condensers direct from the arc. Rays reflected by the lamp house do
not pass through the condensers in the same direction as those coming
directly from the crater and will not focus with them. Hence, the
farther the arc is from the condensers, the smaller will be the
percentage of light used; the shorter the focal length of the
condensers, the closer to them must the arc be maintained, and the
greater will be the percentage of light used. But if the light is
brought too close, there will be undue heating of the condensers and
these, especially the one nearest the light, will be likely to break. So
great is the heat produced that sometimes the two lenses are partially
melted and welded together. This is a frequent occurrence in cases where
very heavy currents are used. It must be recalled that the heat produced
is proportional to the square of the current and that other things being
equal, 80 amperes would produce four times the heat of 40 amperes.
Condenser breakage is quite an important subject and one upon which
there is much argument among operators. Many of the theories held are,
however, not plausible enough to merit mention. The principal cause is
no doubt overheating without allowing sufficient room for expansion in
the setting. No lens should ever be set so that it does not move freely
even while it is hot. Even if free while cold, the expansion, where the
heating is great, may be sufficient to tighten it in the casing, and
this is likely to cause breakage. The best methods of preventing heating
are: a large lamp house well ventilated and condensers of such focal
length as to allow the arc to be maintained at some distance from them.
Drafts of air are often given as the cause of breakage, but the truth of
this is rather problematical. There is no doubt that sudden contraction,
due to rapid cooling, would have a strong tendency to break them; but
the air in operating rooms is not often cold and is not likely to
strike the lens anyway. It must be noted that it is usually the inner
lens, which is ordinarily enclosed, that breaks.
[Illustration: FIGURE 21.]
In the projection of moving pictures there are two important points that
must always be considered. (1) the size of the spot on the gate at which
the film appears, and (2) the clearness of the field or light on the
screen. By properly adjusting the arc, we can make the spot any size we
desire; and the smaller we make it, so long as it covers the whole
aperture, the brighter the light will be. But if we make this spot too
small, we shall bring in the fringe of color which always appears at the
outer edge. Color of this kind is objectionable and must be avoided as
much as possible; but it is not necessary to go to extremes. A little
coloring will not be noticed by the audience and will therefore not be
objectionable. With a given system there will thus be a certain size of
spot which gives the best results obtainable. Considering that if the
spot is increased in size, the light becomes clearer but also less
intense; and that if the spot is decreased in size, the light on the
screen, though more brilliant, is liable to show coloring, a good
operator should practice distinguishing the coloring and make himself as
proficient in this art as possible. The customary proportions of spot
and aperture are shown in Figure 21.
Coloring appears, however, from another cause also, viz., improper
centering or adjustment of the arc lamp with reference to the
condensers. If the arc is not properly adjusted, bands of color such as
are indicated in Figure 22 may appear in any of the positions shown.
This is commonly spoken of as the “ghost”, and it must be eliminated. It
is not possible to get rid of it entirely, but by a little skill,
patience, and experience, it can be reduced to a negligible amount. When
the spot is right and the screen clear, the picture may be focused by
adjusting the objective lens.
[Illustration: FIGURE 22.]
To focus sharply, it is advisable to move the lens in one direction
until the picture appears a trifle blurred; then move it in the opposite
direction until at this point there is also a blurred picture. The
exact focus will be at a point half way between the two. To focus the
lens in this manner is important where the slide or film has some play,
as when the aperture plate on a machine is worn and allows the film some
movement.
_Current Required._--The measurement of the candle power of arc lamps
has never been satisfactorily taken, and the difficulties encountered in
determining it for a projecting arc are especially great because only a
small part of the total light can be utilized and this is constantly
varying. The light may, however, be assumed as proportional to the
wattage of the arc, hence, we can best judge it by noting the volts and
amperes. Where a very strong light is desirable, the arc is usually
drawn out to some length; and as there is a rise in voltage, with a long
arc, in such a case, the light increases at a greater rate than the
amperage. In ordinary projection work, the arc is kept quite short
because of the better definition obtainable by the use of such an arc;
and we may assume that the light obtained is nearly directly
proportional to the amperage. This relation of light and the current
input to the lamp will be practically correct, especially if the size of
the electrodes chosen is proportional to the amperage.
_Current Required for Projecting._--The value of the current to be used
for projection is a matter of some dispute among operators and probably
much of this is caused by the absence of ammeters, most operators merely
guessing at what they are using, or being guided by markings of
rheostats or compensators. In most cases something like 40 amperes seems
to be the rule.
In order to give the reader a clear understanding of the theoretical
requirements, Table I has been prepared. This table is not intended to
act as an accurate guide, but merely to show the amperage theoretically
required with different sized pictures, to bring about the same
illumination in each case.
TABLE I.
CURRENT REQUIRED FOR DIFFERENT SIZE PICTURES.
-----------+--------------+------------------------
Greatest | | Amperes
Dimension | Area +----------+-------------
of Picture | Illuminated. | Direct | Alternating
in feet. | | Current. | Current.
-----------+--------------+----------+-------------
5 | 39 | 8 | 12
6 | 56 | 11 | 16
7 | 77 | 15 | 22
8 | 100 | 20 | 30
9 | 127 | 25 | 37
10 | 157 | 31 | 45
11 | 189 | 38 | 57
12 | 224 | 45 | 67
13 | 260 | 52 | 78
14 | 307 | 60 | 90
-----------+--------------+----------+-------------
Two errors are very common in the computation of the light intensity for
a given picture: (1) the length of throw governs the amperage; and (2)
the amperage depends upon the actual space to be illuminated. Apparently
only an oblong square of exactly the proportions of the aperture in the
machine is illuminated, but in reality the light must be spread out so
that its total illumination covers a circle enclosing the actual visible
picture. This is illustrated in Figure 23 where the enclosed oblong
square represents the space illuminated on the screen and the circle
represents the area over which the light must be spread. The portion
shown by shading is nearly equal to the clear portion and shows that
half of the light is wasted since it is blocked out by the cooling plate
in the machine or the framework of the slides. With increasing size of
picture, the light is, however, diminished in proportion to the area of
the circle and not in proportion to the area of the picture. If, for
instance, the picture were to retain its width and be reduced in height
by one half, or even more, there would still be about the same quantity
of illumination required. For this reason we have, in Table I, given
only the maximum dimension of the picture and have based the amperage
calculation upon the area of the circle which encloses the picture.
[Illustration: FIGURE 23.]
The values given are less than are generally used for small pictures and
more than are generally used for large pictures. As a rule much light is
wasted on small pictures because the apparatus is at hand to deliver it;
with large pictures, the illumination is often poor because transformers
and rheostats are seldom fitted to deliver more than 60 amperes. Much
light can easily be wasted if the picture is made too bright. In such a
case, much of the light is reflected back to the auditorium and this in
turn makes the picture appear less bright.
In determining the amperage necessary to show a picture properly, the
following conditions must be borne in mind, any one of which may
appreciably affect the result:
(1) _Nature of Screen._--A good screen will reflect more light than a
poor one.
(2) _Size of Picture._--The larger the picture, the more light will be
required.
(3) _Character of Film._--Some films are very dark and require extra
illumination.
(4) _House Illumination._--In some cities the law requires fairly bright
illumination of auditoriums and this makes the picture appear less
bright.
(5) _Atmosphere._--Where the air is full of dust, or where smoking is
allowed; much light will be absorbed.
(6) _Lenses._--Some lenses are badly discolored and absorb much light.
(7) _Electrodes and Electrode Setting._--This is a very important factor
and one which a good operator will never neglect.
_Selection of Lenses._--Upon the proper selection of lenses depends very
much the quality of the picture. The size of the picture, under given
circumstances, depends entirely upon the focal length of the objective.
With a given distance between lens and screen there is practically but
one size of picture obtainable. If we wish to obtain a picture of
another size by the use of the same lens, this can be done only by
sacrificing the definition and had better not be attempted.
Very large pictures are desirable only in large halls in which portions
of the audience are very far from the screen. Such a picture requires
very much light and, on account of its size, shows many imperfections to
those who sit in the front rows. It is better to limit the size of the
picture to one which can be easily illuminated, and thus avoid such
imperfections.
TABLE II.
MOTION PICTURE LENSES.
TABLE SHOWING SIZE OF SCREEN IMAGE WHEN MOVING-PICTURE FILMS ARE
PROJECTED.
Size of Mat opening 11-16 × 15-16 inch.
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
E.E. | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 60 | 70 | 80 | 90 |100
In. | ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
2-1/8| 4.8| 6.4| 8.0| 9.6|11.3|12.9|14.5|16.1| | | | |
| 6.5| 8.7|11.0|13.2|15.4|17.6|19.8|22.0| | | | |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
2-1/2| | 5.4| 6.8| 8.2| 9.6|10.9|12.3|13.7|16.4| | | |
| | 7.4| 9.3|11.2|13.1|14.9|16.8|18.7|22.4| | | |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
3 | | 4.5| 5.7| 6.8| 8.0| 9.1|10.3|11.4|13.7|16.0| | |
| | 6.2| 7.7| 9.3|10.9|12.4|14.0|15.6|18.7|21.8| | |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
3-1/2| | | 4.9| 5.8| 6.8| 7.8| 8.8| 9.8|11.7|13.7|15.7| |
| | | 6.6| 8.0| 9.3|10.6|12.0|13.3|16.0|18.7|21.4| |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
4 | | | 4.2| 5.1| 6.0| 6.8| 7.7| 8.5|10.3|12.0|13.7|15.4|
| | | 5.8| 7.0| 8.1| 9.3|10.5|11.6|14.0|16.3|18.7|21.0|
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
4-1/2| | | | 4.5| 5.3| 6.2| 6.8| 7.7| 9.1|10.6|12.2|13.7|15.4
| | | | 6.2| 7.2| 8.4| 9.3|10.5|12.4|14.5|16.6|18.7|21.0
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
5 | | | | | 4.8| 5.4| 6.1| 6.8| 8.2| 9.6|10.9|12.3|13.7
| | | | | 6.5| 7.4| 8.4| 9.3|11.2|13.0|14.9|16.8|18.7
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
5-1/2| | | | | 4.3| 4.9| 5.6| 6.2| 7.4| 8.7| 9.9|11.2|12.4
| | | | | 5.9| 6.7| 7.6| 8.4|10.2|11.9|13.6|15.3|17.0
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
6 | | | | | | 4.5| 5.1| 5.7| 6.8| 8.0| 9.1|10.3|11.4
| | | | | | 6.2| 7.0| 7.7| 9.3|10.9|12.4|14.0|15.6
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
6-1/2| | | | | | | 4.7| 5.2| 6.3| 7.3| 8.4| 9.6|10.6
| | | | | | | 6.4| 7.1| 8.6|10.0|11.4|13.0|14.5
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
7 | | | | | | | 4.4| 4.9| 5.8| 6.8| 7.8| 8.8| 9.8
| | | | | | | 6.0| 6.6| 8.0| 9.3|10.6|12.0|13.3
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
7-1/2| | | | | | | | 4.5| 5.4| 6.4| 7.3| 8.2| 9.1
| | | | | | | | 6.2| 7.4| 8.7|10.0|11.2|12.3
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
8 | | | | | | | | | 5.1| 6.0| 6.8| 7.7| 8.5
| | | | | | | | | 7.0| 8.1| 9.3|10.5|11.6
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
=Example=: With a lens of 5-1/2 inch focus at a distance of 35 ft. the
screen image will be 4.3×5.9; at 40 ft., 4.9×6.7; at 45 ft., 5.6×7.6;
etc.
=Note=: When ordering lenses, give size of picture wanted, and distance
from machine to screen.
TABLE III.
STEREOPTICON LENSES.
TABLE SHOWING SIZE OF SCREEN IMAGE WHEN LANTERN SLIDES ARE PROJECTED.
Size of Mat opening 2-3/4 × 3 inches.
------+---+----+----+----+----+----+----+----+----+----+----+----+----
E.F. |15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 60 | 70 | 80 | 90 |100
In. |ft.| ft.| ft.| ft | ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.
------+---+----+----+----+----+----+----+----+----+----+----+----+----
5 |8.0|10.8|13.5|16.3|19.0| | | | | | | |
|8.8|11.8|14.8|17.8|20.8| | | | | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
5-1/2|7.3| 9.8|12.3|14.8|17.3|19.8| | | | | | |
|7.9|10.7|13.4|16.1|18.8|21.6| | | | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
6 |6.6| 8.9|11.2|13.5|15.8|18.1|20.4| | | | | |
|7.3| 9.8|12.3|14.8|17.3|19.8|22.3| | | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
6-1/2|6.1| 8.2|10.4|12.5|14.6|16.7|18.8| | | | | |
|6.7| 9.0|11.3|13.6|15.9|18.2|20.5| | | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
7 |5.7| 7.6| 9.6|11.6|13.5|15.5|17.5|19.4| | | | |
|6.2| 8.3|10.5|12.6|14.8|16.9|19.0|21.2| | | | |
------+---+----+----+----+----+----+----+----+----+---+----+----+----
7-1/2|5.3| 7.1| 8.9|10.8|12.6|14.4|16.3|18.1| | | | |
|5.8| 7.8| 9.8|11.8|13.8|15.8|17.8|19.8| | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
8 | | 6.6| 8.4|10.1|11.8|13.5|15.2|17.0|20.4| | | |
| | 7.3| 9.1|11.0|12.9|14.8|16.6|18.5|22.3| | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
8-1/2| | 6.2| 7.9| 9.5|11.1|12.7|14.3|16.0|19.2| | | |
| | 6.8| 8.6|10.3|12.1|13.9|15.6|17.4|20.9| | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
9 | | 5.9| 7.4| 8.9|10.5|12.0|13.5|15.1|18.1|21.1| | |
| | 6.4| 8.1| 9.8|11.4|13.1|14.8|16.4|19.8|23.1| | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
9-1/2| | 5.6| 7.0| 8.5| 9.9|11.4|12.8|14.2|17.1|20.0| | |
| | 6.1| 7.6| 9.2|10.8|12.4|14.0|15.5|18.7|21.9| | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
10 | | 5.3| 6.6| 8.0| 9.4|10.8|12.2|13.5|16.3|19.0|21.8| |
| | 5.8| 7.3| 8.8|10.3|11.8|13.3|14.8|17.8|20.8|23.8| |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
12 | | | 5.5| 6.6| 7.8| 8.9|10.1|11.2|13.5|15.8|18.1|20.4|
| | | 6.0| 7.3| 8.5| 9.8|11.0|12.3|14.8|17.3|19.8|22.3|
------+---+----+----+----+----+----+----+----+----+----+----+----+----
14 | | | | 5.6| 6.6| 7.6| 8.6| 9.6|11.6|13.5|15.5|17.5|19.4
| | | | 6.2| 7.3| 8.3| 9.4|10.5|12.6|14.8|16.9|19.0|21.2
------+---+----+----+----+----+----+----+----+----+----+----+----+----
16 | | | | | 5.8| 6.6| 7.5| 8.4|10.1|11.8|12.5|15.2|17.0
| | | | | 6.3| 7.3| 8.2| 9.1|11.0|12.9|14.8|16.6|18.5
------+---+----+----+----+----+----+----+----+----+----+----+----+----
18 | | | | | 5.1| 5.9| 6.6| 7.4| 8.9|10.5|12.0|13.5|15.1
| | | | | 5.6| 6.4| 7.3| 8.1| 9.8|11.4|13.1|14.8|16.4
------+---+----+----+----+----+----+----+----+----+----+----+----+----
20 | | | | | | 5.3| 6.0| 6.6| 8.0| 9.4|10.8|12.2|13.5
| | | | | | 5.8| 6.5| 7.3| 8.8|10.3|11.8|13.3|14.8
------+---+----+----+----+----+----+----+----+----+----+----+----+----
22 | | | | | | | 5.4| 6.0| 7.3| 8.5| 9.8|11.0|12.3
| | | | | | | 5.9| 6.6| 7.9| 9.3|10.7|12.0|13.4
------+---+----+----+----+----+----+----+----+----+----+----+----+----
24 | | | | | | | | 5.5| 6.6| 7.8| 8.9|10.1|11.2
| | | | | | | | 6.0| 7.3| 8.5| 9.8|11.0|12.3
------+---+----+----+----+----+----+----+----+----+----+----+----+----
=Example=: With lens of 10-inch focus at a distance of 20 ft. the screen
image will be 5.3×5.8; at 25 ft., 6.6×7.3; at 30 ft., 8.0×8.8; at 50
ft., 13.5×14.8.; etc.
Table II shows the size of picture obtainable from films, and Table III,
the size obtainable from lantern slides. Since the slide pictures must
be shown upon the same screen as the film, it can be seen from the
tables that lenses of different focal length must be used for the two.
The aim should be to get the two pictures to match as nearly as
possible, but as they are not of the same proportions, it is impossible
to match them exactly in both directions. The nearest approximation that
can be brought about by standard lenses is illustrated in Figure 24. The
heavy lines show the dimension of the picture projected through the
film, and the light and dotted lines show the dimensions obtainable by
the use of slides. If the slide picture is matched to the height of the
film, it will be considerably narrower; if it is matched to the sides,
it will be considerably higher. It would of course be possible to trim
down slides so that the dimensions of the two pictures would be exactly
alike; but as most all stereopticon slides belong to traveling actors
this is not practicable.
[Illustration: FIGURE 24.]
If the focal length of a lens is not known, it can easily be measured by
focusing some distant object, an incandescent lamp for instance, against
the wall of a room or against some screen placed upon a table as shown
in Figure 25. In the case of a single plano-convex lens, the
measurements must be made from both sides--first one side turned toward
the light, and then the other. There will always be some difference
between the two measurements and we must take the mean of the two. To
get the measurement accurately, place a rule upon a table and set up
some suitable object upon which the picture can be projected. Turn the
flat side of the lens toward the screen and focus some distant object by
moving the lens to a point at which the object selected will appear
clearly upon the screen. Note the distance of the flat side of the lens
from the picture. Now turn the lens half way around and focus again in
the same manner, noting this distance also. Add the two measurements and
divide by two; this will give the focal length of the lens. In the case
of an objective lens, we must turn the side which bulges out most toward
the screen and focus in the same manner.
[Illustration: FIGURE 25.]
With the objective lens we have two possible focal lengths to consider.
If we measure from the center of the lens to the screen, we shall obtain
what is called the _equivalent focal length_ (usually abbreviated E.F.
or e.f.). If, instead, we take measurements from the face of the lens
nearest the screen, we shall obtain what is termed the _back focus_, or
b.f., of the lens. In all cases it is important, when ordering, to state
which of the two is meant.
Lenses may also be tested for chromatic and spherical aberration.
Chromatic aberration is the fault of showing colors unduly. It is
impossible to avoid a fringe of color when using only a single lens, but
where we have a complete optical system, consisting of two condensers
and an objective, it must be possible to adjust the combination so that
practically no color is visible. Spherical aberration is best tested for
by laying out very accurately, as in Figure 26, a set of small squares
upon some material that will not be damaged by the heat of the
lamp--mica for instance--and projecting this upon the screen. If the
lenses are all good, the lines will all appear square; if the lenses are
poor, the lines will appear curved a little, or perhaps considerably.
[Illustration: FIGURE 26.]
The diameter of the ordinary condenser lens is 4-1/2 inches. Smaller
lenses than this cannot well be used because they would not cover the
diagonal of lantern slides. A very common focal length of condenser is
6-1/2 inches. There is no very accurate relation necessary between the
focal length of condenser and objective. There is considerable
difference of opinion on this subject and much of it is induced by the
possibility of condenser breakage which is increased by using
condensers of short focal length, but in this case, as in many others,
the operator must find out by his own experiments.
A very good plan--since, on account of breakage, extra lenses must be
carried anyway--is to carry two 7-1/2-inch and two 6-1/2-inch condensers
and experiment with these. The two of the same diameters may be tried
together and also those of different focal lengths, using the one of
shorter focal length either in front of or behind the other.
HINTS ON MANAGEMENT OF PROJECTING ARCS.
Before starting to work about the lamp, be sure the switch is off.
See that the lamp house is clean and spark tight.
The gauze provided at the top must be kept free from dirt and carbon
ash, or the house itself may get too hot.
The house should be of such dimensions, relative to the length of
electrodes used, that the latter cannot touch either at the top or
bottom and thus ground the circuit on the lamp house and possibly burn a
hole in it.
See that your lamp mechanism is well aligned so that electrodes center
at all positions.
All of the screws and adjustments should be well lubricated frequently.
The heat in the lamp house soon evaporates all lubrication.
Where lamps are used much and carry heavy currents, the leading in wires
will probably need reconnecting about once a week. It is best to
reconnect them some time before they burn off rather than be obliged to
do this during a show.
See that your polarity is right. With direct current, the upper
electrode will retain its heat longer than the lower if connections are
made properly. With alternating current the polarity is immaterial.
Always point your electrodes, especially the lower. If the lower
electrode is not pointed, it will interfere with the light of the
crater.
The recommendations for sizes of upper and lower electrodes vary
somewhat but run mostly to 5/8 inch for upper and 1/2 inch for lower.
The size depends very much upon the current used. If the electrodes are
too large, the arc will travel around the outside and yield a poor and
uneven light.
Always use cored carbons for alternating current.
The best length for electrodes is about 6 inches, if they do not strike
the lamp house.
Notch the carbon electrode a little before attempting to break it off.
Many operators are in the habit of watching the arc, opening the
lamp-house door to look at it. Not only is this injurious to the eyes
but it also interferes with proper judgment of the illumination of the
picture. A better way is to punch a very small hole in the lamp house
exactly opposite the arc. Over this opening a lens may be placed, and a
picture of the arc may be thrown against a wall or screen set up for
that purpose. A picture of the arc is also obtainable in another way: If
the lamp is once set exactly right, a cross may be painted at the proper
place on the screen which will indicate the exact point where the arc
should be maintained. The arc will of course appear inverted. Another
method of keeping the arc always in view without inconvenience consists
in arranging a small mirror, at an angle to the peep glass in the door,
so that it will reflect the arc towards the operator.
An adjustable resistance should always be kept in reach so that the
current may be varied to suit different films or stereopticon lamps.
Keep your hands as free from carbon dust as possible. This dust is
responsible for much damage to films.
CHAPTER IV.
MOTION PICTURES.
Strictly speaking there are no pictures of motion. What we see as such
is simply an optical illusion. This illusion is produced by presenting a
series of pictures of an object in a systematic manner, each picture
showing some slight change from the preceding one. If these changes be
all in a certain direction and brought before our eyes in regular order,
we shall perceive the appearance of motion in that direction. Such
pictures may be made by means of photography.
A very simple form of motion picture is made up in the form of a small
book containing a number of leaves that may be run off under the finger
of the holder. If these leaves contain such a series of pictures as is
mentioned above, the holder, on manipulating them properly, will see
motion reproduced quite naturally.
The manner in which the illusion of motion is produced can perhaps best
be illustrated by Figure 27. Here we have an ordinary film, or it may be
any piece of white paper, upon which are drawn a series of black dots as
shown. If this film--the observer being able to see only that part in
the aperture _A_--be drawn downward the length of one section very
quickly; allowed to rest a moment; then, in the same manner, be drawn
down another section; and this process repeated at proper speed, until
the full length of the film has passed the aperture, we shall have
received the impression that the black dot moved from the lower
left-hand corner to the upper right-hand corner of the aperture. In
order that such an illusion might be perfect, we should have to move the
film so rapidly that the eye would not perceive the movement. This is
not possible except with very weak illumination and we should actually,
in the above experiment, receive a blurred impression, because we could
not help seeing the dots while they were moving, and our eyes would
behold a mixture of stationary and moving dots. In order to produce the
impression of perfect motion, it is necessary to shut off the light
during the time that the film is actually in motion. Thus, paradoxical
as it may seem, in order to simulate motion, we must have the object
which is to appear in motion always perfectly still before our eyes.
[Illustration: FIGURE 27.]
In order that we may not notice that the film is out of sight, it must
be moved very quickly. The actual time during which the picture on the
average film is hidden from view, and in which the picture is changed,
is about 1/80th of a second and the time during which the picture is
stationary is about 4/80th of a second.
The possibility of the illusion of motion pictures depends upon a
faculty of the eye known as _persistence of vision_. The eye retains an
impression for something like 1/25th of a second. When an object is in
motion, we see, therefore, not only one position of the object but all
positions of it during the time of persistence of vision. This time
varies somewhat with the intensity of the light or the impression made
upon the eye. If it is equal to 1/25th of a second in the case of a ball
thrown at the rate of one hundred feet per second, then we should see,
instead of one ball, a large number of balls merging imperceptibly into
one another, or, in other words, a streak of balls four feet long. Thus,
in actual life, we obtain from the moving ball but a blurred impression.
We see thus that in order to produce the impression of motion, we must
present the picture to the eye long enough to stimulate it properly; we
must very quickly remove that picture and substitute another differing
to a slight extent from the former; and we must repeat this process a
number of times. The ordinary moving picture film contains 16 pictures
per foot, and is run off at the rate of about 60 feet per minute, so
that in one minute, we see 960 different pictures.
In order to make motion visible, we must bring it within a certain speed
limit. Thus, to show the motion of a swiftly thrown ball in detail, we
must make it appear to move more slowly than it really does; and to show
the development of a growing plant, it must appear to grow much faster
than it actually does. Both of these requirements can easily be
fulfilled by the motion picture camera and the projecting machine.
A man, walking at the rate of three miles an hour, displaces himself
about three inches during the time of the exposure of one picture, or
1/16th of a second. At this rate we obtain the impression of even and
continuous motion unless he be too close to the camera. In order to
obtain pictures of other objects moving at faster or slower rates, we
must take them at intervals in order that the displacement between
pictures will be about the same or at least not any more. This means
that pictures of rapidly moving objects must be taken at short intervals
and those of slowly moving objects, at long intervals. A kernel of corn
develops into a stalk six feet high in about ninety days. If a
photograph of this is taken every day during its growth and these
pictures arranged in proper order, they will be run off at normal speed
in less than six seconds, thus showing us in six seconds the growth
which actually takes place in ninety days.
The motion picture camera enables us not only to produce the illusion of
motion, but to see in detail what actually takes place in connection
with the moving object at any instant. If we take pictures of a running
horse, for instance, at short enough intervals, we shall be able to see,
on the films, just how he holds or places his feet or any other part of
his body at any time.
In order to obtain a perfect picture simulating motion, we must present
the first picture long enough to stimulate the eye; then we must shut
off the light, remove the first picture, and substitute the second;
remove the second and substitute the third, etc., as long as desired.
During the time that the light is shut off, the first picture must
persist in our vision until the new one has appeared. The two pictures
thus mix until the first one has faded, and thus we obtain the illusion
of motion.
If the bright picture remains too long, the pupil contracts--as
explained in the chapter on Optics--and when next the light is shut off,
the darkness is noticeable and gives rise to the disagreeable phenomenon
of flicker. In order to prevent this over-stimulation of the eye, the
long period of exposure is interrupted by a shutter at least once and,
in some cases, two times; and some machines are equipped with a
three-blade shutter. This three-blade shutter has a wide blade which
shuts off the light while the film is in motion and two narrower blades
which pass across the light during the time that the film is stationary,
to prevent the over-stimulation of the eye.
_Colored Pictures._--There are two general methods of producing colored
motion pictures: One is that of hand coloring or tinting, and the other
is what is known as the _Kinemacolor process_. In the latter process, no
color whatever is used on the film; the coloring is supplied by a
shutter with a green and a red blade which are alternately thrust into
the light by which the picture is projected upon the screen.
In order that this process may be used, the film pictures must first be
taken through screens of corresponding color. The film in the
Kinemacolor camera, or projecting machine, must run at more than double
the speed of that which is used in the ordinary process; and each
alternate picture must be photographed through a red screen; the others,
through a green screen.
The red screen will allow only red light to pass; hence, any part of an
object that contains no red will not affect the photographic emulsion.
Similarly, the green screen will allow only green light to pass; and
such parts of the objects as contain no green will not affect the
emulsion. The alternate sections of film will thus be entirely
different from each other.
In order to reproduce the original color of the object upon the screen,
it is but necessary to arrange that the pictures shall in turn be
projected through the same or similar color screens. In order to
accomplish this, the Kinemacolor machine has, in addition to the regular
shutter which cuts off the light during the time the film is in motion,
an additional two-wing shutter which inserts the properly colored
screens before each picture, as it comes to a standstill in the film
window. Thus we see in alternation, a red picture and a green.
Persistence of vision, which is explained in Chapter IX, helps us to mix
the two colors and we see the object approximately in its own colors.
The color effect is not very good, owing partly to the fact that only
two colors are used. If the three primary colors--red, blue, and
yellow--could be used, the effect would doubtless be better; but the
complications would be multiplied.
In order to get the best results with the Kinemacolor process, colors of
a certain shade are used and the size and depth of coloring in the two
shutters is variable. One of the colors is adjustable and must be so
arranged that when the machine is run without film, it will throw
approximately white light upon the screen. The high speed with which the
film must be run makes it impossible to turn the machine by hand and it
is always motor-driven.
The colored shutters are constantly in the light and absorb a large
portion of it and this must be compensated for by an extraordinary high
amperage. This process requires more than two times as much current as
the ordinary projection of black and white pictures. In order to obtain
the greatest possible amount of light, operators usually run a very long
arc and this often results in imperfect definition.
In arranging for an exhibition, it is important that the film and the
colored screens be correctly placed with reference to each other; to
facilitate this, an identifying mark is placed on the side of one of the
colors.
CHAPTER V.
THE MOTION-PICTURE MACHINE.
A diagrammatic sketch of the essentials of a good motion picture machine
is given in Figure 28. This does not represent any machine in particular
and no machine exactly like it will be found; it does, however, show the
theoretical elements necessary to the projection of motion pictures and
the usual safety devices with which standard machines are provided. As a
variation from Figure 28, the exact method of threading the film through
the Edison machine is given in Figure 29. In Figure 28, the parts are
designated as follows:
_A_--feed reel or upper reel
_B_--feed reel magazine or upper magazine
_C_--magazine fire traps, or film valves, or fire valves
_D_--upper steady-feed sprocket
_E_--presser rollers, or friction rollers, or idlers
_F_--upper feed loop or upper loop
_G_--film-steady drum or film steadier
_H_--film gate
_I_--tension spring
_J_--automatic fire shutter
_K_--revolving shutter
_L_--intermittent sprocket
_M_--lower feed loop
_N_--lower steady-feed sprocket
_O_--take-up reel and lower magazine
_P_--framing device or adjusting lever (not shown)
_Q_--film shields
[Illustration: FIGURE 28.]
[Illustration: FIGURE 29.]
In addition to the above, there are the gearing and the belting which
transmit motion to the various sprockets and drums shown. The whole
function of the machine, however, is the proper moving of the film along
the lines indicated in the figure, the heavy black line representing the
film. The film is unwound from the upper reel by the upper steady feed
sprocket _D_. After forming the upper loop _F,_ it passes over _G,_
through the film gate _H,_ to the intermittent sprocket _L_. This
sprocket moves the film by an intermittent motion allowing the film to
remain stationary in the light for about four-eightieths of a second and
shifting it during about one-eightieth of a second. After leaving the
intermittent sprocket, the film forms the lower loop and then passes to
the lower steady feed sprocket, which prevents the take-up from pulling
the loop away from the intermittent sprocket. The object of the upper
and lower loops is to lighten the work of the intermittent sprocket as
much as possible by making it unnecessary to move anything but the film
between the two loops.
[Illustration: FIGURE 30.]
All of the sprockets and the shutter _K_ are connected together by a
train of gears (see Figure 30, which is the gearing of the Motiograph
machine) and when properly adjusted they all work in proper relation to
each other. The upper steady-feed sprocket feeds just as much film into
the upper loop as the intermittent sprocket takes away; while the lower
steady-feed sprocket takes away just as much as the intermittent
sprocket feeds to it. The film is unwound from the upper reel and
rewound upon the lower; but before it is used again, it must be rewound
from the lower reel upon another one. If it were to be exhibited from
the lower reel without this rewinding, the pictures would be exhibited
backwards.
DISCUSSION OF PARTS.
_A--Upper, or Feed Reel._--The feed reel is usually either 10 or 12
inches in diameter. A 10-inch reel accommodates 1,000 feet of film and a
12-inch reel, about 2,000 feet. The reel fits loosely upon bearings in
the upper magazine and the film is unwound from the reel, which
revolves, by the upper steady-feed sprocket. To prevent its unwinding
more film than is wanted, a small spring is arranged to cause a slight
friction. It is best to keep good reels in the operating room for use as
feed reels. A reel to be used for this purpose should be perfectly true
and in good order; reels sent out from the exchanges are often bent or
have loose parts which cause trouble. A good operator will keep a supply
of good reels always on hand.
_B--Upper Magazine._--The upper magazine is a steel box made up without
solder and fitted with a steel door on the crank side of the machine. It
exists for the purpose of protecting the film against fire or injury
from other causes. It is important that the door be kept closed for,
without this precaution, there is no fire protection. In some cities, it
is required that the door be provided with spring hinges to keep it
closed, but it is doubtful whether this is a wise provision; for since
the door must be opened to change films and since there is nearly always
great haste in making the change, it is likely that the operator will
block the door open if the spring hinges interfere with him. All doors
now swing to the side, but it seems as though it would be a great
improvement if the doors were arranged to drop down. The door would
then, even if open, prevent fire from below reaching the film. The
magazine might as well be entirely missing as to be left with the
side-swinging door open. Many operators have the habit of opening the
door to watch the progress of the reel instead of noting it on the
screen, or of preparing for a change of reel by opening the door long
before necessary and starting the new run before closing the doors.
Needless to say, this is highly reprehensible.
In order to put out a fire in case it should communicate to the reel,
the magazines have been connected with water piping and a valve so
arranged that the water could be turned on instantly thus filling the
magazines. If the door of the magazine is kept closed, the progress of a
fire will be comparatively slow.
_C--Fire Traps._--The fire traps are an important adjunct. Their object
is to prevent fire, which often occurs at the aperture, from reaching
the film on the reel. All of the various makes of fire traps have been
tested and will ordinarily prevent fire from passing through them. The
larger the roller and the smaller the space around the film, the better
the traps seem to be. The metal of the rollers has a cooling effect
upon the flame and this is undoubtedly one reason why they do their work
so well. None of them, however, is absolutely sure.
If the opening through which the film passes is made too narrow, the
film is likely to brush one side or the other and wear grooves in it, or
cut entirely through it, with the result that a splice may be caught in
the opening and the film torn. Fire traps, cut in this manner, have been
the cause of many fires. This trouble is due mainly to improper
alignment of the magazine with the fire traps. Examine them often and,
if the least wear shows, improve the alignment.
It is advisable that every operator test his traps with pieces of film
and assure himself that they will not, under ordinary circumstances,
carry fire; but he must never rely too much upon them as safeguards, for
although they will check a small blaze, such as would result from the
burning of a few inches of film, they would probably not extinguish a
fire occurring when there was much film crowded around them, as might be
the case when a take-up reel or the intermittent sprocket failed to
work. If the film catches fire and there is any possible chance to do
so, the operator should tear it off at the upper and lower magazines,
and thus break the communication. But if there happens to be a lot of
film lying loosely about, it is advisable for the operator to get away
as quick as possible.
_D--Upper Steady-Feed Sprocket._--The office of the upper steady-feed
sprocket is to unwind the film from the reel and feed it toward the
intermittent sprocket. In order that it may do its work well and relieve
the intermittent sprocket of all unnecessary strain, there must be a
loop _F_. The upper steady-feed sprocket is in continual steady motion
and feeds just as much film to the loop as the intermittent sprocket
takes from it by periodical jerks. The main trouble with all sprockets
is in the wear of the teeth, as in time they wear away near the body of
the sprocket and form hooks. The best sprockets have a number of teeth
to engage the film. In some of the older machines, only two teeth catch
the film, in which case, if two holes in the film are torn out, the film
may stand still.
_E--Presser or Friction Rollers._--The presser or friction rollers exist
merely to hold the film in its proper place. They are sometimes spoken
of as friction rollers although they have nothing to do with friction.
_F--Upper Loop._--The upper loop is provided for the purpose of storing
slack film which the intermittent sprocket may pull away with a very
rapid jerk. It avoids placing unnecessary strain upon the film. The
upper loop often causes trouble by enlarging; this occurs when the
intermittent sprocket fails to work while the steady sprocket continues
to feed. The most frequent cause of this is due to faulty films. In
order to prevent the excess film in the upper loop from falling over in
front of the light of the arc lamp (which would quickly set it on fire),
the film shields _Q_ are provided. In many machines they are too short
or too narrow to be of much use.
_G--Film-Steady Drum, or Film Steadier._--The film steadier is not found
in all machines. If the upper portion of the machine head is properly
arranged, the film may feed directly into the gate.
_H--Film Gate._--The film gate is to hold the film in position. In
order that the picture may be properly projected, the film must lie
perfectly flat and at a fixed distance from the lenses. If this distance
varies, there will be improper focusing of the picture. The film must
also be held in its proper position laterally and vertically. The film
gate has nothing to do with the height of the film. This is taken care
of by the framing device. The film gate wears quite rapidly and when
badly worn allows the film too much play. In order to avoid the wobbling
of the picture, a new gate must be provided. There should be
considerable metal surrounding the aperture and there should also be an
air space and some little ventilation. The metal surrounding the gate is
subject to the heat from the arc lamp rays and, unless properly
constructed, may overheat and damage the tension springs, if it does not
set fire to the film.
_I--Tension Springs._--The tension springs are provided to hold the film
flat against the film gate and also to check its motion as soon as the
intermittent sprocket has stopped pulling it. A certain intermittent
movement is advertised to make the change from one picture to another in
1/96 of a second at normal operating speed. The film moved at this rate
of speed acquires considerable momentum; and the office of the tension
spring is to bring it to rest as quickly as possible. If the springs are
set too tight, they will cause the machine to run hard and may also be
the cause of tearing out splices. They should be set just tight enough
to keep the picture steady; anything beyond this will merely cause
unnecessary wear of the parts, besides calling for unnecessary exertions
on the part of the operator.
_J--Automatic Fire Shutter._--The automatic fire shutter is provided
for the purpose of shutting off the light when the machine is not in
motion. It is required by most city ordinances. The ideal fire shutter
would be one so controlled by the film that it would remain up only
while the film is moving at operating speed. The shutters in use at the
present time vary widely in details of construction. Some of them are
raised and admit the light to the film the instant the handle is pressed
sufficiently to start the machine. Other types are raised only after the
machine has nearly attained its proper speed. Some operate as indicated
in the figure and others operate from the side. All of them are liable
to become deranged at times, and it is no unusual thing to find them
tied up in some way because they failed to work properly. The shutter is
a great convenience when in proper order and has probably prevented many
fires; but it is good practice to consider it as entirely absent and not
get into the habit of relying upon it. The operator should always push
his lamp to the side whenever anything is out of order. This act is
necessary in most houses at the end of every run of film showing
stereopticon slides. It would inconvenience the operator but little to
do this at all times, and then the habit would be formed and become
second nature. On the machines now in use, there is no automatic shutter
governed by the action of the film, which would give protection in case
the film should come to rest, as old films often do when their sprocket
holes are torn out, or, as often happens, when the film splits along the
row of holes. In some machines the fire shutter is made up of such thin
material and rests so close to the film that the light may heat it
sufficiently to fire the film below it. Pieces of film often become
detached at the aperture and remain in the light long enough to be
ignited. Against such occurrences as these there is no protection
whatever except the watchfulness of the operator. If the film fits
snugly into the gate and is well enclosed, such fires do not usually
spread.
_K--The Revolving Shutter._--The revolving shutter is sometimes arranged
in front of the machine and often between the film and the lenses. The
shutter _K_ is of the “barrel” type and is further illustrated in Figure
31 at the left. The object of the shutter is to shut off the light
during the time that the film is in motion so that the impressions made
upon the eye by the succession of stationary pictures may not be blurred
by the motion of the film when it is changed. The ideal shutter is one
that shuts the light off instantly and, at the expiration of the
necessary time, allows it as quickly to come to view again. The
barrel-type shutter allows the light to pass through while it is in the
position shown by solid lines, Figure 31, and has it entirely shut off
when in the position indicated by broken lines. When it is in motion,
the upper wing begins to shut off the light from the top and the lower
from the bottom, thus causing the total eclipse of light in one-half of
the time that a single wing shutter could do it.
[Illustration: FIGURE 31.]
At the right of Figure 31, we have a cone shutter such as is used in the
Motiograph. The stem and gearing of this shutter are set at an angle of
forty-five degrees to the rays of light, for the purpose of arranging
them inside of the mechanism without taking up too much room. There are
two cones of the kind illustrated in Figures 32 and 33 and they move in
opposite directions, thus shutting off the light in about the same time
that a barrel shutter could be made to do it. These two figures show the
positions of wings just before shutting off the last vestige of light
and just as they are beginning to admit it again.
[Illustration: FIGURE 32.]
[Illustration: FIGURE 33.]
The disk type of shutter is much used. It may have either one, two, or
three blades and may also be double, i.e., two disks revolving in
opposite directions so as to shut off the light from two sides of the
opening at once. Three types of disk shutters are shown in Figures 34 to
36. In order to obtain a flickerless picture, it is necessary to cut off
the light not only during the time that the film is in motion, but also
during part of the time that it is standing still. If the light were not
interrupted during the time of exposure, the light interval would be so
long that the difference between the dark period, when it is shut off
entirely, and the light period would be great enough to be noticed in
the form of a flicker.
[Illustration: FIGURE 34.]
[Illustration: FIGURE 35.]
[Illustration: FIGURE 36.]
If a single-blade shutter is used, it must make two revolutions during
the time that a single picture is exposed; once to shut off the light
while the film is being moved and once to interrupt the period of
illumination. This shutter would thus have to run twice as fast as the
two- or three-blade shutter, the three-blade shutter interrupting the
light twice during the time the film is standing still.
The three-blade shutter has one wide blade which must be in front of the
light while the film is moving and two narrower ones which interrupt the
light during the time of exposure.
The two-blade shutter must have both blades of the same size and run one
and one-half times as fast as the three-blade shutter to obtain the same
effect, i.e., cause two interruptions of light during the time of
exposure.
In order to shut the light off very rapidly with a blade, which
approaches the opening from one side only, the disk shutter must be of
sufficient size to produce great angular velocity. It can not well be
used, therefore, inside of the mechanism, but is usually placed in front
of the machine; and it is much larger than the aperture which it must
cover. The shutters must all be set so as to shut off the light during
the time that the film is in motion. When the dividing line between two
film pictures is in the center of the aperture, the shutter should have
it entirely covered. It is possible, however, to arrange the shutter so
that the film may move a trifle before the light is shut off and
continue in motion also for a very short time after the light is again
admitted. This is practicable with the “Geneva” intermittent movement
which starts and stops the film gradually.
A shutter should be adapted to the speed of the film movement. There is
no need of keeping the film covered any longer than it is in motion. A
quick film movement and a narrow shutter will add considerably to the
light obtained from a given lamp and a given current consumption. If the
shutter is not properly adjusted, there will be what is known as “travel
ghost”, “light rain”, or “halo”. These are due to the improper timing of
the shutter, allowing a part of the picture to be seen while in motion.
The travel ghost may be seen either at the top or the bottom, according
to whether the shutter is set too fast or too slow.
[Illustration: FIGURE 37.]
[Illustration: FIGURE 38.]
_L--Intermittent Sprocket._--The intermittent sprocket is a very
important part of the machine. Owing to the swiftness with which it
strikes the film, the teeth on it are more subject to wear than those on
the other sprockets. The number of teeth that engage the film is also an
important item. In order to obtain the best picture with the least
expenditure of light, the film should be moved very rapidly so as to
allow the greatest possible length of time for the stationary picture.
The film should, furthermore, be started slowly; then increased in
speed; and the speed should decrease gradually until brought to rest,
and thus avoid unnecessary jerking. It is also necessary to prevent all
motion of the film during the time the mechanism which moves it is
preparing for the next succeeding movement. All of these conditions are
admirably fulfilled by the “Geneva” movement, Figure 37. The pin wheel
_W_ is in continuous motion and the pin is so placed upon it that it
enters one slot of the cross and carries it along with it, thus causing
a quarter revolution of the cross each time the pin wheel makes one
revolution. The cam band _P_ is cut away sufficiently to allow the cross
to make a quarter revolution, but the remaining portion of it is made to
fit the cross snugly, so that when the cross is not in motion it is held
rigid. Figure 37 shows the movement just starting and Figure 38 shows it
half completed. It can be seen that the motion begins very slowly; comes
to a maximum when it is in the middle; and ends slowly; thus subjecting
the film to the least possible strain. This movement is widely used and
may be arranged with one pin, as shown, or with two. If the pin wheel is
equipped with two pins, it will move only half as fast as with one pin
and thus the proportionate time that the film is in motion will be
lengthened. By making the pin wheel large as compared with the cross or
star, the time during which the motion takes place can be reduced as
much as desired; but the characteristic feature of starting and stopping
the film gradually will be lost directly in proportion as the relative
size of the pin wheel is increased.
[Illustration: FIGURE 39.]
The “Geneva” movement requires extremely accurate construction and
careful management. If dusty or insufficiently lubricated, it wears very
rapidly. It is often arranged so that it can be immersed in oil while
running. Figure 39 shows a practical application of the “Geneva” as used
in the Motiograph. The cover shown at the right entirely encloses it.
[Illustration: FIGURE 40.]
[Illustration: FIGURE 41.]
[Illustration: FIGURE 42.]
[Illustration: FIGURE 43.]
A form of intermittent movement which has lately come out and is used on
the Powers No. 6 Cameragraph is shown in Figures 40 to 42 inclusive. The
moving element of this intermittent movement is a diamond-shaped
projection on the surface of a revolving disk. The driven element is a
cross carrying four pins--termed a “pin cross”--and is indicated by
broken lines. The raised portion _A_ of the diamond strikes one of the
pins of the cross and starts it in motion as indicated in Figure 41.
The movement of the cross can be traced by the black dot indicating one
of the pins. At every revolution of the driving disk, the cross makes
one quarter of a revolution and when not in motion is held rigidly
locked by the locking ring _R_, which fits snugly between the pins. This
movement probably moves the film in the shortest time of any. It is
entirely encased, runs in oil, and is practically noiseless. The manner
in which it is installed in the machine is indicated in Figure 43.
_M--Lower Feed Loop._--The lower feed loop is provided to prevent the
lower sprocket from pulling on the film and thus interfering with the
work of the intermittent sprocket. The intermittent sprocket feeds the
loop, by jerks, three-fourths of an inch of the film at a time; and the
lower sprocket is geared to take away that much by steady motion. Thus,
everything working properly, the loop remains of the same size. In case
the lower sprocket is not working properly, the loop may become enlarged
and the film may run onto the floor. Sometimes the intermittent sprocket
does not work properly and the lower sprocket pulls away all of the
loop.
_N--Lower Steady-Feed Sprocket._--The lower steady-feed sprocket
maintains the loop and restrains the film. If it were not for this
sprocket, the take-up mechanism would pull directly on the intermittent
sprocket and keep the film jiggling.
[Illustration: FIGURE 44.]
_O--Take-up Reel._--The take-up reel is identical with the upper reel
and, in fact, the two are generally used interchangeably. In both cases,
only perfect reels should be used; and reels delivered by exchanges are
very often untrustworthy.
The principle of the take-up mechanism is shown in Figure 44. When a run
of film is started, the diameter of the roll of film being small, the
reel must travel comparatively fast to take up the film as fast as it is
unwound from the upper reel. As more and more film is gathered by the
lower reel, the travel must become slower until finally, with a
twelve-inch reel, the number of revolutions per minute are but about
one-fourth as many as at the beginning.
In Figure 44, _A_ is the reel which is fastened to the spindle _B_; _C_
is a collar also fastened to the same spindle. _D_ is a loose pulley
capable of revolving around the spindle without imparting motion to it
and arranged so that it can be driven by a belt. _E_ is a spiral spring
which is controlled by the small collar _F_, and it presses the loose
pulley against the collar _C_. If the loose pulley is pressed against
_C_ with sufficient force, it will cause _C_ to revolve with it; and the
movement of _C_ will impart motion to the reel and wind up the film
which is fastened to it. _D_ must revolve fast enough so that, even when
the reel is empty, it will take up all of the film fed to it. As the
film is wound up, the size of the roll of film increases and the speed
must become less; thus _D_ begins to slip a little on _C_ and this
slippage increases until the end of the run.
Friction in one form or another is the only method by which take-up
reels are now operated; but in some of them this friction may be in a
belt, as well as in the two disks as shown. Sometimes a fiber washer is
interposed between the two friction disks. In the Motiograph take-up,
Figure 45, the necessary friction and slip are all in the belting.
[Illustration: FIGURE 45.]
If the take-up is not handling the film properly, it is due to a loose
belt or to insufficient tension on the spring. The take-up probably
causes more trouble than any other part of the mechanism and it should
be carefully watched. On account of the trouble incurred from it, there
are still many operators who prefer to run films into a tank; although
the take-up saves them much time. A careful operator who understands the
care of machinery would have little trouble with a take-up; while the
one who has not the faculty of keeping things in order would, perhaps,
be better off (fire hazard neglected) with a tank. With most machines,
the take-up magazine is located where it is very difficult for the
operator to watch its operation. Those machines in which it is located
in easy sight of the operator have considerable advantage in this
respect. A careless operator is often compelled to open the lower
magazine door and turn the reel by hand when the take-up tension is out
of order. Remember that the take-up can cause trouble only when it is
out of order and that a good operator will never allow it to get out of
order and he will inspect it carefully before every show.
_P--Framing Device._--The aperture in the film gate is just large enough
to fit the picture on the film. If the film is accurately placed, the
whole picture will show upon the screen. In order to bring this about,
the machine must either be provided with a means of adjustment, or a
certain part of the film must be placed upon a certain sprocket tooth.
This latter method causes considerable annoyance and waste of time;
furthermore, every splice in a film would have to be made exactly right
or the whole succeeding part of the show would be “out of frame”. In
order to avoid these troubles, every machine is provided with a
so-called framing device. Somewhere about the machine, convenient to the
operator, will be found a lever by which, during the operation of the
machine and without interruption, the position of the film with
reference to the film window and the lens may be adjusted either up or
down. The precise manner in which this is done varies with different
machines. The framing device must be capable of adjusting the film to
somewhat more than the width of one picture.
The splice should be so made that framing becomes unnecessary and this
is accomplished by cutting out full pictures so as to leave each picture
with the full four holes. If a picture is left, for instance, with only
two holes, it will appear upon the screen with the black line, which
should be hidden in the center. If this happens, by moving the framing
lever either up or down, the picture can again be centered upon the
screen.
[Illustration: FIGURE 46.]
_Q--Film Shields._--The film shield is an important part of the machine;
although it is often very much neglected and in many machines is so
small as to be practically useless. It should prevent films from
crowding out into the light when the loops enlarge. Many operators do
not take kindly to machines that enclose the film too tightly; for in
case the take-up fails to work properly, it is often a convenience to be
able to handle the film. This is also the case where very badly torn or
old film is used. Many old films are so badly worn that they will not
run properly on the sprockets and must be very closely watched.
[Illustration: FIGURE 47.]
[Illustration: FIGURE 48.]
It is not thought necessary or advisable to go into the details of
construction of any particular machine as changes are made nearly every
year so that such description would not be of great value and might, in
fact, be misleading. Any person possessing a little mechanical ingenuity
and taking an interest in such matters can readily learn the
peculiarities of any of the machines. They are, after all, when one has
mastered the principles, quite simple affairs. All progressive
manufacturers, furthermore, publish instruction books which are issued
whenever a change is made in any machine; and these are always
obtainable.
[Illustration: FIGURE 49.]
In Figure 46 we have a general view of the Edengraph machine. It will be
noticed that the total length of film between the two magazines is
exposed to view making threading of the film very easy; also that the
revolving shutter works between the objective lens and the film gate.
The well-known Edison machine is shown in Figure 47. This is one of the
best known machines; and it is very compact and durable.
[Illustration: FIGURE 50.]
The Motiograph is shown in Figure 48. The take-up, shown on the lower
magazine, is different in this machine from what it is in the others.
This machine also has a special rewind feature. By an adjustment of the
operating handle, the film, after having been run off onto the take-up
reel, can be rewound upon the upper reel without taking it out of the
magazines.
Figure 49 shows a general view of the Powers No. 6 Cameragraph
mechanism.
The Simplex mechanism is shown in Figure 50. In this machine the film is
entirely enclosed between the two magazines.
CHAPTER VI.
THE FILM.
_Film Making and Characteristics._--All film is of celluloid. The
commercial film used for the projection of motion pictures comes in long
strips, is about one and three-eighths inches wide, and about one
two-hundredths of an inch thick. Upon one side of this film is a
photographic emulsion of gelatine impregnated with a solution of bromide
of silver. This silver is sensitive to the light and has the property of
turning black, during the process of developing, in proportion to the
quantity of light that has struck the different parts of the emulsion.
Wherever the light has been very intense or has been applied for a
sufficiently long time, the film, after treatment with a proper
developing fluid, becomes very black; where no light has reached it, it
remains white; and in the intermediate portions, the shading is in
proportion to the light that has reached those parts.
Film that has not been exposed and developed is of a creamy yellowish
color and, if immersed in a solution of hyposulphite of soda, will turn
perfectly clear and offer no obstruction to the light. If, on the other
hand, it is completely exposed and developed, it will become solid
black; such black pieces are sometimes used as leaders and tails in
connection with motion pictures.
The emulsion on the film melts at a temperature of about 90 degrees
Fahrenheit and will then run off the film. It is also soluble in warm
water. The celluloid of the film is very inflammable and bursts into
flame at a temperature of 284 degrees Fahrenheit. If it is ignited while
tightly wound upon a reel, it burns somewhat slowly; if it is ignited in
a tank, the beginning of the fire is also slow; but as soon as the air
in the tank attains the proper temperature, the whole of the film bursts
into flame at once. It acts in this respect very much like gunpowder. In
burning, the film gives off a dense brownish mass of poisonous smoke.
Film will burn even when it has no access to the air and a film fire
cannot be put out by smothering.
Before sending out their films, the manufacturers treat them with
glycerine. This is for the purpose of keeping them pliable as long as
possible. In time, however, a film dries out and there is then very
little that can be done with it. At any rate, whatever is to be done
should be done by the exchange, as no operator has facilities for
handling it.
Some films are waterproofed and a little water will not damage them
much; but other films must be kept dry. If by accident a roll of film
should become wet, it can be saved by very quickly unrolling it and
spreading it out to dry. If it can be spread out quickly enough, it will
not be harmed; but if it is not spread out at once, the emulsion will
stick to the celluloid and will come off when the film is finally
unwound.
There is also a process of fireproofing film. This must not, however, be
too thoroughly relied upon, especially if the film is old. Fireproof
film is a valuable aid, but it is best to treat it exactly like other
film in respect to the fire hazard.
On account of its inflammable character, film should always be kept in
fireproof enclosures.
The film contains sixteen pictures to each foot and for every picture
there are four sprocket holes on each side of the film. The size of the
aperture plate and also the size of the actual picture on the film is
11/16 inch high by 15/16 inch wide. If the picture on the screen is
enlarged to 18 feet 9 inches in width and 13 feet 9 inches high, every
part of the picture on the film will appear 57,600 times as large on the
screen as it is in the film. In viewing a 1000-foot film, the audience
will see 16,000 separate pictures following each other so rapidly that
the change from one to the other will be unnoticeable. If a three-blade
shutter is used, the beholder will see the light shut off and turned on
48,000 times in the same run of film, but will not be able to notice it
if the speed with which it is done is correct.
New film coming directly from the exchange is likely to be too moist and
cause trouble from that condition; while old film is likely to be too
dry and cause trouble by warping or bending out of shape.
_Patching Film._--All reels of films contain patches and but little
trouble will arise from them, provided they are properly made and
reasonable care is exercised in watching them. In order to obtain a
satisfactory splice, it should not be made more than 3/16 inch long; a
greater length than this will reach two sprocket teeth and this is
objectionable because the splice stiffens the film and will prevent it
from fitting the teeth properly. The splice should be so made that it
does not throw the pictures out of frame. If made properly, it will
never be noticed in the running picture at all, unless a large quantity
of film is cut out. If however, the splice be made over part of a
picture--as would be the case if an improper number of holes were cut
out--the picture, when projected, would appear out of frame, i.e., the
dividing line between two pictures would then appear on the screen and
would continue to show until the framing lever had been adjusted to
cover the dividing lines again. Care must also be taken that the film is
straight after being spliced. If the ends are not fitted squarely to
each other, there will be a bend in the film at the point the splice is
made; and this may cause trouble.
Many operators have a plate of ground glass fitted into a work bench at
the place where splices are to be made and a small lamp under this
glass. This is very convenient for, as a rule, the illumination in an
operating room is very poor although a good light is necessary to match
film properly. If ground glass is used, the rough side must be at the
bottom; otherwise it will be difficult to clean off the cement which
adheres to the glass.
By making every splice in exactly the same order, the operator will
acquire the habit of doing it nicely and quickly. Most splices are made
in a hurry (before or after a show) and in this, as well as in all
things that must be done hastily and at a moment’s notice, habit and
practice are very important. One does not become expert until he has
practiced it often enough to be able to do it mechanically.
[Illustration: FIGURE 51.]
Proceed as follows and as outlined in Figure 51: Cut exactly along the
dividing line of one picture and save this to become the top piece. (The
emulsion side of the two sections of film must always be either up or
down, and for convenience it is well to form the habit of having them
always one way or the other.) Now, with a sharp knife, scrape the
celluloid side of this piece until all of the gloss is removed for a
width of a little more than 3/16 inch. Do not scrape any more than
necessary, but be sure that all dirt and oil are removed. Now take up
the other part of the film and cut off the bad portion, leaving a strip
a trifle less than 3/16 inch beyond the dividing line. This strip need
not be scraped on the back, but all of the emulsion must be scraped from
the front side of it. To scrape the front side nicely, lay a short ruler
across it to act as a guide for the knife and scrape up to this ruler.
Scrape only to the dividing line, but give particular attention to the
sprocket holes and the edges, for it is at the edges and sprocket holes
that film first begins to work loose and where it is most difficult to
secure proper adhesion. Both parts of the film should be so scraped
that, when finished, each will furnish half of the dividing line. Now
apply a liberal coating of cement to the latter section and carefully
lay the first upon it, taking great care that the sprocket holes in the
two sections overlap and match perfectly. It is for this matching
purpose that the glass in the table (with a small lamp underneath it)
is valuable. The sprocket holes must match accurately and the films must
line up straight. After the upper film is placed, it must be pressed
down for a few seconds until the cement has set. Wipe off all
superfluous cement before it has time to set. Be sure that there are
four sprocket holes for each picture. Keep a knife on hand and use it
for no other purpose. Keep also a suitable stone on which to sharpen
this knife, for, unless the knife is very sharp, it will do only poor
work and the edge on it will not last long at any rate. Ordinary cement
will not hold non-inflammable film, and water-proof film must be more
thoroughly scraped than ordinary film. In the bench, provide a recess
into which the cement vessel may be set and where it will always remain.
Provide also a pair of scissors for cutting the film.
[Illustration: FIGURE 52.]
The Edison film mender is shown in Figure 52. This is made with three
gates or hinges. Those on the right and left clamp down to hold the
film, while the narrower one in the center clamps down on the cemented
edge. To mend a film, place one of the pieces to be joined in the
mender. Place that part of the gage marked “cut” in position, cutting
between first and second perforations above the floor line separating
the picture. The remaining piece of film should be placed in the other
side of the mender and cut exactly on the line. Set the opposite end of
the gage marked “scrape” on the first section; remove the emulsion; cut
to the floor line with a sharp pen knife, moistening it so that the
emulsion will come off easily. After applying the cement with a brush to
the scraped piece, lay the other piece over it in the mender, clamping
it down; and leave it for an instant to adhere.
CHAPTER VII.
GENERAL HINTS ON INSTALLATION, OPERATION, AND CARE OF MACHINES.
_Tools._--First of all provide a suitable and sufficient set of tools.
There should be an assortment of screw drivers, large and small, to fit
the different sizes of screws used not only with the machine but about
the operating room and with other devices as well. Keep these screw
drivers filed or ground to proper shapes. A screw driver rounded off, or
too small, or bent, will ruin the heads of the screws. There should also
be a collection of pliers (especially such as are commonly termed “gas
pliers”), files, raps, chisels (for wood and metal), punchers, drills,
and monkey wrenches; a hack saw, a blow torch, and a vise will also come
in handy. Tools, however, depend upon the man who uses them. There are
men who would be no better off with such a collection at their disposal;
on the other hand, a clever man would be able to make use of all these
tools and show his skill thereby.
_Aligning the Machine._--The first thing to do in setting up a new
machine, after it has been unpacked and put together, is to line it up
properly. This can be done by stretching a string from the lamp house,
through the center of the condenser opening, film gate, and objective
lenses. These must all be in such a position relative to one another
that a taut string will pass through the center of each. The center can
be determined by measurement either with a suitable rule or a pair of
calipers.
_Anchoring the Machine._--The next step is to anchor the machine so as
to reduce vibration to a minimum. Vibration is often due to a flimsy
floor which will not stand the strain of the unavoidable vibration and
thus helps to increase it. Often, however, the vibration is due to some
part of the running gear of the machine, such as the revolving shutter,
for instance, being out of balance. If there is undue vibration, the
machinery should be looked after first, for it is much better to prevent
the vibration by removing the cause than to seek to restrain it by bolts
and braces. If the machine runs very badly, it cannot be bolted down
with any satisfactory results.
_Inspection._--After the machine is lined up and running smoothly, the
next step should be to inspect every part of it thoroughly for loose
screws, pins, gears, or other parts. It is best to do this by beginning
at one end and continuing to the other so that no part may be
overlooked.
_Upper Magazine._--See that the spring in the upper magazine, or on the
door of it, which prevents the momentum of the upper reel from running
off film after the sprocket has ceased pulling, is in good order and the
reel true. A crooked reel will generally cause trouble and force the
operator to keep the magazine door open so as to be able to aid the
movement of the film.
_Fire Trap._--The fire trap must next be looked after. The upper
magazine, the fire trap, and the sprockets must all be in perfect
alignment and the film must run through the trap without touching either
side. Film wears the metal very fast and a day’s run with the film
brushing one side of a valve will show decided wear. If it is allowed to
run this way, it will soon cut through the side of the valve, forming
an opening in which the film will run and in which it will often catch
and tear splices. Special care must be exercised to see that the traps
are kept clean and, when not in use, they should always be covered. The
film is drawn through the traps very rapidly and any kind of dirt
accumulating on them is apt to scratch it. After a fire, burned film is
likely to be found in the trap, and this, when cold, becomes hard and is
difficult to remove.
_Cleaning Machine._--After the machine has been used for a considerable
time, it is advisable to wash out the gears thoroughly. Not only the
gearing, but the bearings also, should be reached by this process. If
one has plenty of gasoline and a suitable receptacle, the quickest way
is to take out the lenses and immerse the whole machine for a while in a
vessel filled with gasoline. It can then be taken out, worked a little,
and immersed again; this process being repeated until all of the old and
hardened grease is worked out. If facilities for immersing the machine
are not at hand, the bearings may easily be cleaned by using kerosene or
gasoline to oil them and then running the machine a little; this will
work all of the old grease out of the bearings. The gearing can be
cleaned in the same way. If gasoline is used, it will quickly evaporate
and leave the machine in good order.
_Oiling Machine._--After the machine is thoroughly cleaned, it may be
oiled again. The best lubricants to be used range all the way from
sewing-machine oil to beeswax. Beeswax is used only for the gearing and
only where that is somewhat worn. It is solid enough to compensate for
some of the wear. That part of it which works out on the sides should
be wiped off at once. There are many lubricants to be recommended; in
general, however, the particular kind to be chosen depends upon the
parts to be lubricated. A part moving rapidly with no great friction can
be well served with a light lubricant, but heavier parts, or parts that
fit so closely as to cause much friction are best served by some
lubricant that has more body. If a light oil is used, it will of course
have to be applied oftener than a heavy oil. None of the extremely light
oils should be used because they run off too fast.
_Care of Sprockets._--Having thus cleaned and oiled the machine, the
next step is to look after the sprockets. In an old machine it is likely
that the sprocket teeth will be so worn as somewhat to resemble hooks. A
little wear of this kind will do no great harm, but beyond a certain
point of wear, the sprocket teeth tend to carry the film farther than is
necessary and in so doing tear out the holes. Some sprockets are made
reversible and the trouble can be remedied by turning the sprocket wheel
around. In most cases it will, however, be advisable, when the old teeth
are badly worn, to get a new set of sprockets.
The sprockets must also be examined for accumulation of dirt, for in
case there is much dirt on a sprocket, it will not guide the film
properly. The dirt will cause it to jump, and thus bring about an
unsteady picture; or, if the accumulation of dirt is sufficient, it may
even cause the film to jump off the sprocket. Hundreds of feet of film
have been ruined in this way. The sprockets should be cleaned every day
with a suitable brush. An operator too careless to clean his machine
properly, is also as a rule too careless to watch the picture properly;
and he suddenly finds himself with a length of film ruined by sprocket
teeth which have cut a row of holes through the center of it.
The sprocket should be made to align perfectly with the upper and lower
magazines, the fire valves, the film gate, and the path of the film.
They may be aligned with a straight edge made of a flexible piece of
steel or, if nothing else is available, a piece of film may be placed
upon the sprockets. Be sure to examine the film carefully before using
it for this purpose; old film is apt to be warped. The film must fit
perfectly in the aperture and on the different sprockets. The sprockets
must have little or no end play, especially the intermittent, or it may
cause the picture to wiggle sideways. The idlers, or presser rollers,
should not fit tight enough to bind, but must fit closely enough to hold
the film securely. In the adjustment of these and all other parts of a
moving picture machine, considerable practice and judgment are necessary
to estimate how much freedom of movement is permissible.
_Adjusting Intermittent Sprocket._--The “Geneva” wears rapidly when it
is not well oiled or protected; and it becomes necessary to adjust it.
There are other ways of doing this but generally it is done by means of
an eccentric bushing which must be set to compensate for the wear. In
many operating rooms, there is a wide range of temperature and, as heat
expands the metals, they will bind unduly when they become warmed up, if
the star and cam wheel are set too close while cold. In many machines
the heat from the lamp warms them up irrespective of the temperature in
the operating room.
_Automatic Fire Shutter._--Some means of adjusting and regulating the
automatic fire shutter is usually provided; this should be adjusted so
that it will not rise before the machine is in motion and will fall
before it has come to rest. Under no conditions should the stationary
film be allowed in the path of the light. There are fire shutters that
do not depend upon the motion of the machine, but are raised as soon as
the handle is pressed to give motion to the machine. This type of
shutter is raised as soon as the handle is pressed and may expose the
stationary film to the light. Such a shutter should not be tolerated at
all. The fire shutter should be carefully tested to see that it operates
properly under all conditions. If one is not acquainted with the
particular type of fire shutter in use, it is advisable for him to test
it with the lamp turned on it for some time. Some shutters are so thin
and are placed so close to the film they are supposed to protect, that
it is possible to fire the film through them.
_The Revolving Shutter._--In some machines the light is shut off before
the film begins to move, and in others a very slight movement of the
film is permitted before the shutter closes out the light entirely. The
general object of the shutter is to shut off the light while the film is
moving and to interrupt the long period of light so that it may not
affect the eye too strongly and give rise to the sensation of flicker.
The exact manner of setting the shutter varies with different machines,
but in general the above consideration must be kept in view. Where a
shutter has several blades, it is always the larger one that shuts off
the light while the film is moving and this is the only one to which any
attention must be paid while setting.
The best test for the setting of the shutter is the appearance of the
picture. The best kind of picture with which to judge a shutter is one
that has strong contrasts of light and darkness. If a picture contains a
very bright object, the travel ghost will appear if there is any motion
while the light is on the film. If the shutter is set too late, the
travel ghost will be in the lower part of the screen; and if too soon,
in the upper. If the shutter is made too small, the travel ghost cannot
be entirely eliminated but must be divided half on the lower and half on
the upper side of the picture. The travel ghost and other imperfections
of a picture are much more noticeable to persons close to the screen
than to those far away. As the operator himself is the farthest away of
all spectators, he is likely to miss many imperfections that are plainly
visible to the audience. For this reason many operators have opera
glasses through which they view their pictures.
_Framing._--When the machine is finally threaded, the film should be so
placed on the upper sprocket that, with a proper loop, the picture will
be in frame when the framing lever is about in the middle position. This
position will allow the operator to frame either up or down without
seriously affecting the loop. The framing is best done by looking at the
picture thrown upon the screen.
_Film Gate._--The film gate needs special attention when new film is
being used. Very often new film is so damp that some of the emulsion
sticks to the gate. Wet film running through a machine makes it work
hard and is likely to cause much trouble. With such film it is often
necessary to stop during a run and clean off the gate and it is always
necessary to do so after each run. Never use a sharp tool or anything
that may scratch the gate or roughen the bearing surfaces where the film
runs. The gate must be watched closely for wear; if worn too much, the
picture may be out of focus or jump in all directions.
_Tension Springs._--The tension springs will repay careful study by the
operator. The object of the springs is twofold: First, to hold the film
flat, and second, to introduce sufficient friction to overcome quickly
the slight momentum which the film has acquired while in motion. The
springs should be just tight enough to do this. The pressure of the
springs necessary to overcome the momentum of the film is proportional
to the speed at which the film moves. If the film is to be run at a high
rate of speed--as is often the case when a crowd is waiting outside the
doors--the tension must be increased, unless it was originally great
enough to hold the film at this rate of speed.
As the film varies in thickness, the adjustment cannot be made too
close; but the springs must be made tight enough so that they will hold
the film at the highest rate of speed when the thinnest part of the film
is passing through. If the springs are not sufficiently tight, the
picture will seem to move a trifle. They must be just tight enough to
prevent this at the highest rate of speed at which the machine is to
operate.
_The Take-Up._--The lower sprocket exists for the purpose of
restraining the take-up from pulling on the intermittent sprocket. There
is thus actually always a strain that the one puts on the other. For
this reason the tension on the take-up should be no greater than
necessary. Excessive tension will increase the wear on the machine; may
cause the tearing of many splices; and will cause the machine to run
hard, thus imposing extra labor on the operator. While the lower reel is
nearly empty, the strain is small; but as it fills up, the strain
becomes greater. The driving wheel or belt must move at a fixed rate of
speed while the slip of the driven wheel becomes greater with every
turn. Thus the energy expended in friction becomes greater as the
picture progresses. Defective belting, crooked reels, or poor adjustment
of tension are the most frequent cause of annoyance in the take-up and
the take-up as a whole gives probably more trouble than any other part
of the equipment.
_Cover._--Every machine should be covered when not in use and the cover
should be dust tight.
_Getting Ready for the Show._--The first thing for the operator to do
before entering the operating room is to put away all smoking
paraphernalia he may have about him. No smoking tobacco or matches
should ever be found inside of any operating room. After taking this
precaution the next thing will be to rewind and inspect the films to be
used for the next performance. If the reel happens to be properly wound
when received, it is not absolutely necessary to rewind it and, in an
emergency, one may take a chance and run it through without previous
inspection. When possible, however, the operator should always inspect
his film before using and also familiarize himself with its contents.
A film is best inspected by rewinding it slowly from one reel onto
another, gripping it with a gentle pressure, and allowing it to run
between the fingers. This method will catch all splices and also all
split films. Films are frequently split along the line of the sprocket
holes. The film should be run through the fingers in this manner twice:
once to catch such irregularities as make themselves manifest along the
outer edge and again to catch the defects that exist only in the middle
of the film. Every splice should be carefully inspected to note how the
sprocket holes match and whether the splice will throw the picture out
of frame. All splices that are not properly made should be re-made as
directed in Chapter VI.
If there is sufficient time, it would be well to run the film through
the machine in the regular way. Thus the operator can familiarize
himself with the picture and note every place where improvements can be
made by adjusting the speed, for very often a picture may be improved by
slowing down or increasing the speed in certain scenes. The flicker
varies with the illumination; the brighter the light the greater will be
the tendency to flicker, if the machine is not run at proper speed. The
operator should know before hand just when his brightest scenes will
come on so as to adjust the speed or, in case he must run slowly, so as
to reduce the flicker by reducing the illumination. Running the film
through will also show every place where the picture goes out of frame
and all such places can be corrected. All bad places should be cut out
of the film, but not more than necessary. Every additional section cut
out of a film will increase the jerkiness of the motion. A few are not
noticed, but one often sees in old reels, scenes that are simply
ridiculous because too much film has been cut out. Such effects are
especially noticeable if a picture is run slowly.
The operator must always be prepared to run pictures at abnormal speeds.
It is bad practice to do so but often unavoidable; for instance, if the
next number on the program is delayed, it is desirable to extend the
picture until it is ready in order that the audience may not be
dissatisfied; or if all the seats are filled and people are waiting
outside, the manager may demand extra speed so as to make room for the
outsiders.
The operator who is wise and has had experience will provide himself
with a receptacle in which he can store the pieces he cuts out of films.
Saving such pieces and taking them to the exchange will often make an
impression upon the exchange management and induce greater care in the
inspection of films at the exchange. Such pieces will sometimes be good
evidence in the hands of an operator, too, in case he is charged with
having damaged a film. While the film is being run through the machine,
it can also be measured. Every turn of the crank means one foot of film.
Before the film is wound upon the reel from which it is to be exhibited,
an opaque piece of film about two feet in length should be attached to
the tail end of the film. This piece of film will in some cases serve a
double purpose. With some machines the film can be rewound from the
lower magazine into the upper without taking it out of either, if this
tail piece is long enough to remain fast to the upper reel after the
whole film is run off the upper reel. Thus the operator when through
running the picture may immediately change his crank and, without delay,
begin to rewind. The main purpose of this tail piece, however, is to
shut off the light immediately after the last section of the picture has
been exhibited. If the light is left on the screen while the operator
slows down the machine, it will cause a disagreeable flicker to appear
before the eyes of the audience immediately after the picture is over,
and thus leave an unpleasant impression.
The beginning of the film should be provided with a leader from three to
five feet long. The purpose of this leader is to allow the operator time
to frame his picture and get his lamp adjusted before the title appears,
or is finished, so that when the picture actually begins, everything
will be running smoothly.
The film must be rewound upon the reel in such a manner that when
threaded, the emulsion side will face the lamp. If this is not done, the
titles will read from right to left instead of from left to right as
they should. It will be well for the operator to form the habit of
regularly testing his film by the actual projection to see that his
titles are right. There are often many other things to think of and
cases of absent-mindedness are not uncommon. If the title shows wrong,
the reel may be taken out and turned around; this will reverse it, but
ordinarily it is best to wind it in one certain way.
Every operator should have a full supply of good reels for use in the
machine and should never let these get out of his house. The reels
furnished by the exchanges are often very bad and if perchance a real
good one is sent out, some wide-awake operator will surely keep it.
Although it is best to rewind a reel as slowly as your time will permit,
it is necessary sometimes to do it very rapidly. The reel from which the
film is being unwound should be under a steady tension and some form of
brake is often provided. The practice of “pulling down,” which is
indulged in by so many operators, is highly reprehensible. It causes the
emulsion side of the film to slide over the other and any dirt or dust
that may be between the two is thus ground into them. The scratches thus
caused soon fill up with dirt and cause streaks in the picture which are
commonly termed “rain.” The dirt--especially the carbon dust, which is
so plentiful in an operating room and which is very sharp and cuts badly
into the emulsion or the celluloid--disfigures the picture very much.
A good film box should be located in every operating room and it should
be so conveniently located that the operator will use it in preference
to anything else. Such a box should be provided with automatic
self-closing covers and should be near the floor, as the film is less
liable to dry out there than if placed near the ceiling where the heat
is greater. Nothing whatever should be placed upon the floor itself and
it should be kept absolutely clean. Although it is very bad practice to
run film on the floor and it should not be done, there are occasions
when it will be done and, if the floor is clear of all obstruction, the
hazard of doing so is much reduced. An operator who, having trouble with
his take-up and having run the film on the floor, has had it entangled,
will appreciate this fact. The spasmodic bursts of applause which an
impatient audience is apt to offer while the show is being delayed are
not especially comforting to any one in such a position.
Under no circumstances should film ever be left exposed. Occasional
small fires at the aperture plate are unavoidable and in such cases the
operator will seek to tear off the burning film and throw it on the
floor, and it may then ignite the film lying about. Form the habit of
keeping your film under cover; and make up your mind that it is
absolutely necessary. Keeping films under cover will also tend to save
them from drying out and prevent accumulation of dust.
A slide with the words “Just a moment please,” or some equivalent
expression, will reassure the audience; one should always be kept in
place and when any trouble causes an interruption of the show, the lamp
should be pushed over thus exhibiting this sign. The habit of pushing
the lamp over in case of interruption is also a very good one. It is the
simplest and most natural way of taking the light off the film and
satisfying the audience at the same time. This practice should become a
habit. It will be a great protection against fire in addition to the
other safety devices. All the work of making ready should if possible be
done before a considerable part of the audience is in the house. It is
best to let none of them see the process of getting the show in
readiness.
As has been stated before, an occasional fire at the aperture is
unavoidable. Sometimes pieces of film are torn loose, stick where they
are subject to the light, and in time catch fire. A fire thus started
may easily consume all of the film between the two magazines and will
progress slowly toward the lower one, but towards the upper one very
rapidly. If the doors of the magazines are tightly closed and fit tight
and if the fire traps are in good order, there will be but a small
chance of fire getting at the rolls of film that may be in either
magazine. Every effort should, however, be made to extinguish it before
it reaches the fire traps. Just what can be done depends upon the design
of the machine. In order to be prepared for such an emergency, the
operator should study his machine and decide what would be best to do.
In many cases the film can be torn off before the fire extends to the
magazine and thus break the connection. In some machines the film runs
in such a narrow groove that the fire is generally extinguished there
without any outside help. Some operators have been known to open their
magazines and attempt to remove the film after a fire had started,
resulting in burning all their film. It is far better to leave the film
in the magazine and trust to the fire traps than to attempt to remove
it.
No visitors should be allowed in any operating room during the time that
the audience is in the house, more especially when the time for opening
the show approaches. There should be nothing to distract the operator’s
attention from his duties. The arc lamp should be started a few minutes
before the show is to begin so that the electrodes may burn to their
proper shape and the light be adjusted. Before the show begins, the
house should be made dark for a few minutes. This will make the picture
appear brighter.
In case the film tears during a run, some operators have the habit of
pinning the pieces together with the intention of making the splice
after the picture is run. This practice is well enough for an operator
who feels sure that he will not forget about the pin, but with one who
is forgetful or absent-minded, or who has a lot of other duties which
might take his mind off the film, it is a bad practice. The film may be
rewound with the pin remaining in position and cause very serious
trouble. It is safer to put the film on the take-up without pinning.
In a general way the points alluded to in the following list should be
carefully looked after. This is a list which is used by the Electrical
Inspection Bureau of the City of Chicago in checking up operators and
their equipment.
Theatre address.......................................................
Perm.
Operator’s name .................................................
Temp.
Address.................................. Lic. No. ...................
MOVING PICTURE MACHINE INSPECTION.
An x indicates defect; a check, satisfactory condition.
-------------------------------+--+--------------------------------+--
1 Smoking? | |25 Fan Motor? |
-------------------------------+--+--------------------------------+--
2 Matches? | |26 Guards on inc. lights? |
-------------------------------+--+--------------------------------+--
3 Condition of Films? | |27 Unauthorized persons? |
-------------------------------+--+--------------------------------+--
4 Magazine closed? | |28 Rewind approved? |
-------------------------------+--+--------------------------------+--
5 Film exposed? | |29 Rewind location? |
-------------------------------+--+--------------------------------+--
6 Arc enclosed? | |30 Rheostat, 5 ft. above floor? |
-------------------------------+--+--------------------------------+--
7 Guard fastened? | |31 Rheostat, contacts enclosed? |
-------------------------------+--+--------------------------------+--
8 Upper fire guard? | |32 Rheostat, safe location? |
-------------------------------+--+--------------------------------+--
9 Lower fire guard? | |33 Box for films, approved? |
-------------------------------+--+--------------------------------+--
10 Upper magazine approved? | |34 Box for films, cover? |
-------------------------------+--+--------------------------------+--
11 Lower magazine approved? | |35 Shutters, guides? |
-------------------------------+--+--------------------------------+--
12 Upper magazine, lock well? | |36 Shutt’s self-closing, |
| | supports? |
-------------------------------+--+--------------------------------+--
13 Lower magazine, lock well? | |37 Booth lining? |
-------------------------------+--+--------------------------------+--
14 Light shutter? | |38 Floor? |
-------------------------------+--+--------------------------------+--
15 Fuses enclosed? | |39 Shelving? |
-------------------------------+--+--------------------------------+--
16 Fuses, proper size? | |40 General appearance? |
-------------------------------+--+--------------------------------+--
17 Inc. circuit? | |41 Door, self-closing? |
-------------------------------+--+--------------------------------+--
18 Arc switches, enclosed? | |42 Box office? |
-------------------------------+--+--------------------------------+--
19 Arc switches, control | |43 Unapproved display? |
rheostat? | | |
-------------------------------+--+--------------------------------+--
20 Arc switches, condition? | |44 Main cutout cabinet? |
-------------------------------+--+--------------------------------+--
21 How much open arc wire? | |45 Emergency cutout cabinet? |
-------------------------------+--+--------------------------------+--
22 How much open inc. wiring? | |46 Number emergency lights? |
-------------------------------+--+--------------------------------+--
23 Open contacts? | |47 Fan motors auditorium? |
-------------------------------+--+--------------------------------+--
24 Flexible or BX cord? | |48 Rheostat in base. cond.? |
-------------------------------+--+--------------------------------+--
49 Exits.....? Gas.....? Sperm Oil.....? Electric.....?
----------------------------------------------------------------------
.............................
Inspector.
These defects must be corrected within..............days.
The Electrical Inspection Bureau must be notified as soon as repairs
are made.
CHAPTER VIII.
LIGHT.
Light is emitted from a luminous body. A luminous body is one in which
all of the particles are conceived to be in violent motion, which motion
is transmitted to a supposed ether. The existence of such ether cannot
be demonstrated, but it is supposed to exist because it is impossible to
think of anything being transmitted except through some medium. This
ether is a rather imponderable substance; it is supposed to pervade all
space; and it exists in all matter and in every vacuum. It is supposed
to be elastic without weight and capable of transmitting motion without
loss of energy or friction. It is, however, assumed to be modified to
some extent by the matter in connection with which it exists. Thus the
rate of transmission of light waves is different for air, glass, water,
and other substances through which they may pass and such bodies as are
entirely opaque are supposed to suppress the ether vibrations, resulting
in light entirely.
Light is then a mode of motion of this universal ether which pervades
all space even to the farthest star. The motion of this ether is
conceived to be about as illustrated in Figure 53. If we take a heavy
string or a small rope and, stretching it reasonably taut, jerk it
forward and back quickly a few times, it will be seen to move and assume
the appearance shown in the figure; and that portion shown between the
two vertical lines represents one complete wave.
[Illustration: FIGURE 53.]
The assumption of this ether and the vibratory motions of it form the
only explanation that is capable of accounting for all of the phenomena
of light. All other theories advanced have failed to stand the test;
sooner or later some phenomena have appeared which could not be
explained by them.
Light waves are known to travel through space in straight lines until
they meet with some medium which is capable either of reflecting,
refracting, or absorbing them. The rectilinear propagation of light,
consisting of vibratory motion, is one of the most difficult parts of
the theory to explain. It involves rather deep study and more
mathematics than the scope of this work will warrant. Suffice it to say
that the rectilinear propagation is brought about through the
interference of light waves. An analogy of this can be found in a stream
of water. It is well known that a stream of water issuing from the
nozzle of a garden hose moves in a straight line until gradually forced
into a spray by the resistance of the air. Yet the water passing through
the hose is interfered with on all sides and on all sides there is a
tendency to deflect it. If the water were to move along slowly so that
we could observe the action on one side independently of the balance of
the stream, we should see a series of waves being formed by every
particle of the hose which offers resistance to the flow. The waves
formed in the interior of the hose on all sides are interfered with by
waves from all other directions and the result is motion in a straight
line. In a similar way it may be conceived that the millions of light
waves emitted from a luminous body interfere with each other and thus
cause light rays to move in a straight line.
Light is a form of energy and can be converted into other forms of
energy. It can be converted into heat, for instance, or can be used to
produce chemical effects. Light differs from heat only in the rate of
vibration and the length of the ether waves. Heat can be reflected in
the same manner as light, as the following experiment will show: Arrange
two ordinary reflectors as shown in Figure 54. In the focal point of
one, place a heated iron ball or something of the kind; if this be hot
enough, it will ignite paper suspended in the focal point of the other
reflector, although a thermometer placed anywhere between the two
reflectors will give only a small indication of a rise in temperature.
[Illustration: FIGURE 54.]
All light rays as well as heat rays are in themselves invisible; we can
see only the object which emits or reflects them. If a beam of light be
allowed to enter a darkened room, as shown in Figure 55, we shall
probably be able to see the whole path of the rays illuminated, as well
as the spot on the floor. But this will be because of particles of dust
in the air which reflect the rays to our eyes. If we introduce some
smoke into the room, we shall see the light much more plainly because
there are now more particles of matter to reflect it. On the other hand,
if special precautions are taken to have the air absolutely clear of
dust, we shall be able to see nothing but the spot on the floor.
[Illustration: FIGURE 55.]
Light travels through space at the rate of about 186,000 miles per
second. White light is a combination of light of many colors, but the
speed of transmission is the same for all colors. The length of waves
and the rate of vibrations, however, vary. The red rays have the longest
waves and the slowest rate of vibration; they vibrate about three
hundred ninety-five billion times per second and the wave length is
about 0.0008 millimeters. The violet rays possess a wave length of about
seven hundred sixty-three billion per second. There are light waves
which are longer than the red rays and these are known as _infra red_.
They are not visible to the eye but their existence can be proved in
many ways. Light waves shorter than the violet are also invisible and
are known as _ultra violet_. These have much importance in photography
and to this class belong the X ray.
[Illustration: FIGURE 56.]
We have mentioned above that white light is a combination of light rays
of many colors. This can be proved by the following experiments: If we
arrange to have a beam of sunlight pass through a small hole into a
darkened room, it will pass to the wall on the opposite side in a
straight line and give us white illumination upon a small spot. If we
now arrange a prism in the path of this ray or beam of light, we shall
find that the light no longer passes straight to the wall but that
instead, it is bent in a certain direction and furthermore shows us a
brilliant array of colors. This is illustrated in Figure 56. The rays
are thus shown to be separated into their constituent colors; red is
shown at the top and the following colors merge imperceptibly into one
another--orange, yellow, green, blue, indigo, and finally violet at the
bottom.
The reason for this change is that the rays of light on entering the
glass are slowed down--those of the higher rate of vibration more than
the others. The violet rays are thus said to be more refrangible than
the red, for instance. The colors thus produced are simple colors. This
is proved by the fact that if the light is passed on through another
prism, it will be again reflected but will not be resolved into other
colors; although whichever color is carried to the next prism will
spread out and show finer gradations in its color.
[Illustration: FIGURE 57.]
The colors given above are those obtainable from the decomposition of
sunlight and make up what is known as the _solar spectrum_. If instead
of sunlight some other illuminant be used, the arrangement of colors
will be different; and it has been found possible to tell from the
colors of the spectrum what substances are burning, or heated to a
luminous degree, in the source from which the light comes. This method
is known as _spectrum analysis_.
There are several ways in which light, which has thus been separated
into its fundamental colors, can be re-composed so as to give us white
light again. One of these methods consists in arranging an inverted
prism to receive the light, as shown in Figure 57. The rays leave the
second prism parallel and produce the effect of white light. Another
method consists in gathering the rays from the prism by a lens, as
shown in Figure 58. Furthermore, if we take a disc and paint the colors
of the solar spectrum upon it in the proper proportions, as indicated in
Figure 59, and cause this disc to be rapidly revolved, we shall see it
as almost white.
[Illustration: FIGURE 58.]
[Illustration: FIGURE 59.]
Another fact which goes to prove the undulatory, or motion, theory of
light is that two sources of light arranged to oppose each other can
actually be made to produce darkness. To do this, the waves of one
source of light must be made so that they exactly oppose those of the
other; thus they destroy each other and destroy what light there is in
either. There are other methods, but this can be partially accomplished
in the following manner: Two small mirrors of black glass or of metal
are placed, as shown in Figure 60, very close together and so that they
form an angle of nearly 180 degrees. A beam of light arranged to fall
upon both of the mirrors will be reflected in such a manner that the two
halves interfere with each other and cause bands of light and darkness
to appear. The dark lines are due to the opposition and nullification of
certain of the light waves.
[Illustration: FIGURE 60.]
[Illustration: FIGURE 61.]
The intensity of light diminishes directly as the square of the distance
through which it is transmitted. This is illustrated in Figure 61. The
light, starting from a point, is limited by the size of the first square
at the left; it spreads out more and more, and illuminates larger and
larger spaces. Exact measurement will show that the spaces illuminated
by a ray of light are always exactly proportional to the square of the
distance from the point of light. This law, however, applies strictly
only if the distances considered are long compared to the source of
light, so that the light may be considered as being a mathematical
point, that is, having no physical dimensions. If the source of light,
for instance, were of the same size as the first opening and of uniform
intensity there would result the same intensity of illumination of a
similar space at all distances. There would, however, be an outer fringe
of light which would be proportional to the law of inverse squares. Many
reflectors are arranged to throw very nearly parallel rays; and with
these the intensity remains the same except for absorption, which is
ordinarily not very great.
We see things only through the rays of light they reflect. All colored
bodies have this peculiarity, viz., that they are capable of reflecting
only such rays as make up the color the body is said to possess. A red
body, for instance, absorbs all colors except red and reflects red only.
A black body absorbs all rays and a perfectly dull black body is visible
only by contrast; that is, we do not see it but we are aware that there
is something invisible before our eyes. When we are in a perfectly dark
room, we see nothing but we have blackness before our eyes. A perfectly
white body is one which reflects all of the rays of light and absorbs
none.
When we view things through colored glasses, we see them only in the
colors which the glass will transmit. If we view a red body through a
green glass or under a green light, it will appear black because it is
capable of reflecting only red rays and in the green light there are no
red rays; hence there is nothing to be reflected and the red appears
black.
CHAPTER IX.
PRINCIPLES OF VISION.
Through the medium of our eyes we see objects by means of the light
which is reflected from them. This light enters the eye and forms an
inverted image of the object upon the retina, just as an inverted image
is formed upon the ground glass of a camera. This impression made upon
the eye is corrected automatically, so that, although we see everything
upside down, we are not at all aware of so doing. The proof of this
peculiarity of the eye is found in cases where persons born blind have
later through operations acquired sight. In the eyes of slaughtered
cattle also the image can be seen inverted. A further proof that we are
able thus to adjust ourselves is found in the experience of persons
using cameras with ground glass screens. The image on these screens is
always inverted both horizontally and vertically. The user soon learns
to see his object, although inverted, in the natural way, that is,
vertically, because this is so plain that he must take it into account
at every focusing. He does not, however, accommodate himself to the
reversal from right to left because this is of no consequence ordinarily
and is not noticed. Many photographers, who have been accustomed to the
vertical inversion, still find themselves confused when trying to locate
the right and left of a view seen through the lens.
Unless a special arrangement of lenses is provided, all images cast
upon screens through small openings appear inverted. The reason for this
can be seen from Figure 62. It is obvious that no other light can reach
the bottom of the screen through the pin hole _O_, at the left of the
figure, except that coming from the flame of the candle; also that no
other light can reach the top of the screen except that reflected from
the bottom of the candle at the right. Hence the image of the candle
appears inverted.
[Illustration: FIGURE 62.]
[Illustration: FIGURE 63.]
A general understanding of the structure of the eye can be had from
Figure 63. _W_ indicates a watery substance in the front of the eye; _I_
is the iris which has power to contract or expand and thus regulate the
quantity of light admitted to the eye; _P_ is the pupil; _L_ is the
lens; and _R_ is the retina which connects with the optic nerve and the
brain. The lens is made up of several parts having different indexes of
refraction. The whole resembles an ordinary convex lens but has
considerable power of adjustment. When looking at objects close by, for
instance, the pupil can often be seen to bulge out which is its method
of accommodating itself to objects close at hand.
To the iris falls the duty of regulating the quantity of light which is
to reach the retina. If confronted by a bright light, it closes
partially; in a dim light, it opens out wide. When subject to a
flickering light, there is a tendency to follow the flickerings by rapid
opening and closing, which causes pain. If subject to flickerings long
enough, however, the pain becomes somewhat less, probably because the
iris has come to rest on an intermediate point.
An image formed upon the retina remains for some time, the time varying
with the intensity of the light. Very intense impressions are supposed
to last about one twenty-fifth of a second; milder ones as long as
one-tenth of a second. This tendency to retain images is known as the
_persistence of vision_ and can be noticed in many ways. A twenty-five
cycle alternating current falls to zero fifty times in one second; and
fifty times in each second there is a slight cooling off of the
incandescent filament. Yet the variation in the intensity of the light
is noticeable. Many of the sleight-of-hand tricks depend upon this
persistence of vision and the projection of moving pictures would be
impossible without it.
We are able to judge distance principally through the fact that we have
two eyes. If our eyes were immovable, we should see two images for
every object. But as they are movable and as both normally point
directly at the object we are looking at, their axes form angles with
each other and in this way we are enabled to judge the distance, as well
as other qualities of objects.
When both eyes are centered upon an object, the impressions received by
the brain from both sources are mixed, and the picture we become
conscious of is a composite of the two images in the eyes.
This is verified by the fact that many persons with defective vision can
see much more clearly with one eye than with both. They are not able to
focus both eyes upon the same point and thus the perfectly clear picture
which may exist in one eye is mixed with an uncertain picture in the
other.
In youth normal eyes are able to adjust themselves to different
intensities of light and different distances very rapidly. This power is
largely curtailed as age advances. Where a young person can almost
instantly, after gazing at some distant object, turn to a newspaper and
read, the eyes of a person of advanced age generally require
considerable time before they can adjust themselves in the same way.
Quite frequently, however, very old people regain their powers of vision
and become able to do without the glasses formerly used.
All of the above facts should be thoroughly understood by those having
to do with illumination used by a mixed audience. Light that may seem
perfectly satisfactory to one may be entirely unsuitable for another.
CHAPTER X.
REFLECTION.
Light may be reflected from opaque or transparent bodies such as glass.
In the case of transparent bodies, the reflected rays are not noticeable
unless the ground behind the reflecting body is dark. If there is much
light behind a pane of glass, for instance, the pupil of the eye will be
partially closed and not be able to see the faint light which is
reflected. As we gradually darken the space behind the glass, the image
begins to appear more and more distinct, partly from contrast with the
dark background and partly on account of the increased opening of the
pupil. This can be readily noticed if some evening out of a
dimly-lighted room we look at some object just discernible. If we then
turn on the light suddenly, the object will at once disappear but
reflections will appear in the glass where there were none before.
The reflections from clear glass are much stronger at an angle than when
the rays are thrown straight back. This can be seen by placing any
object directly in front of a pane of glass with a dark background. If
we place the eye so as to receive only those rays which are reflected
directly back, we shall obtain but a weak reflection. If, however, we
place the object a little to one side and stand close to the glass, we
shall see the object almost as plainly as in a regular mirror.
A ray of light is always reflected at exactly the same angle at which
it strikes, the reflecting body; that is, the angle of incidence is
equal and opposite to the angle of reflection. This can be illustrated
by Figure 64. If a mirror be attached to the pointer in the position
shown at an angle of exactly ninety degrees and a beam of light be
allowed to enter through the slit at the top, it will be reflected back
exactly to the spot at which it entered. If we then turn the pointer
slightly, we shall notice that the reflection of the beam of light moves
twice as fast as the pointer and, when the pointer occupies the position
indicated by broken lines, the light will be reflected at right angles
to the line along which it enters. If the mirror is turned still more,
the same law will hold; so that, if the mirror were turned through an
angle of nearly ninety degrees, the reflected beam of light would in the
same space of time make an angle of nearly one hundred eighty degrees.
[Illustration: FIGURE 64.]
Reflected light results in the formation of images in mirrors and other
reflecting bodies and, by bearing in mind the law of reflection given
above, we can readily explain how these images are formed and the manner
in which they appear to us.
[Illustration: FIGURE 65.]
Let _N_, Figure 65 be an object in front of the mirror. The only rays
that are reflected back to the eye are those that strike the mirror at
the proper angle. All others are wasted with reference to the
particular position of the eye. If the eye and the object reflected are
equally distant from the mirror, we need but draw a line at right angles
to the mirror and half way between the eye and the object and, from
these two, draw lines to the point at which the perpendicular line
strikes the mirror. The two lines thus drawn will give us the path of
the incident and the reflected rays. The image will appear to lie in the
direction from which the reflected ray comes and as far behind the
mirror as the object is in front of it. If the eye and the object to be
reflected are not equally distant from the mirror, it is more difficult
to find the paths of the rays and it simplifies matters very much to use
the following construction: Draw a line from the object before the
mirror at right angles to the mirror and extend it behind the mirror as
far as the object is in front of it. From this point behind the mirror,
draw another line to the eye. By drawing a third line from the object to
the point in the mirror where this line, from back of the mirror to the
eye, crosses it, we shall obtain the paths of the rays and the position
of the image in the mirror. The image will exist in the mirror at the
point where the reflected and incident rays meet but will have the
appearance of lying some distance behind the mirror. This is illustrated
in Figure 65, _N_ being the object reflected and _M_ the apparent
position of the image to the eyes as located in the cut.
[Illustration: FIGURE 66.]
[Illustration: FIGURE 67.]
[Illustration: FIGURE 68.]
In Figures 66 and 67 the same construction is used to show the
appearance of arrows as they are reflected from a mirror to the eye.
Objects standing erect over horizontal mirrors or arranged at right
angles to mirrors and looked at, as in Figure 68, always appear
inverted. This can be noticed in quiet ponds of clear water which give
reflections of trees and other objects. Figure 68 shows two arrows, one
horizontal, the other vertical; by the construction in the figure one
appears inverted, the other not. If the arrow were placed in the
position indicated by broken lines, the eye would see only the butt. If
the arrow were placed a little nearer the horizontal, it would appear in
its natural position; if a little more vertical, it would appear
inverted in the mirror.
[Illustration: FIGURE 69.]
All objects seen in mirrors are reversed with reference to right and
left. A pocket on the left side of a person facing a mirror will appear
to be on the right side. Printed matter held before a mirror will appear
just as it would if seen through the paper from the back side and will
have to be read from right to left.
[Illustration: FIGURE 70.]
[Illustration: FIGURE 71.]
[Illustration: FIGURE 72.]
If an object be placed between two parallel mirrors as _B_, Figure 69,
there will be a vast number of reflections visible at the point _D_.
Several reflections of _B_ will come to the eye in the manner indicated
but there will be a large number of additional reflections. If the
mirrors are exactly parallel and absolutely smooth, the number of
reflections would theoretically be infinite. At each reflection,
however, some light is absorbed and some diffused so that many of the
reflections are not discernible. Two mirrors set opposite each other
will also give many reflections of each other as indicated in Figure 70.
The images seen in parallel mirrors all appear arranged in straight
lines on both sides, as indicated in Figure 69. If now one of the
mirrors be inclined so as to form an angle with the other, the long line
of images will seem to become curved and finally lie in a circle. If the
mirrors be placed at right angles to each other, as in Figure 71, there
will be three reflections of the object _C_ visible and these will
reach the eye by the paths shown. If the mirrors be placed at an angle
of sixty degrees to each other, five images will appear as shown in
Figure 72, in which _A_ is the object being reflected.
The following tabulation shows the number of images obtainable at
different angles between the mirrors.
+---------------+-----------+
| Angle between | Number of |
| mirrors | images |
+---------------+-----------+
| 90 degrees | 3 |
| 72 degrees | 4 |
| 60 degrees | 5 |
| 45 degrees | 7 |
| 30 degrees | 11 |
+---------------+-----------+
[Illustration: FIGURE 73.]
Instead of being plain, mirrors may be either _concave_ or _convex_. A
concave mirror is hollowed out in conformity with a small section of the
surface of a sphere. If a piece of glass be cut out of a hollow sphere,
the inner side of it will show the surface of a concave, and the outer
side, the surface of a convex mirror. A section of a concave mirror is
shown in Figure 73. _C_ is the center of curvature and any line drawn
from the surface of the mirror to this center is at right angles, or
normal, to the curvature of the mirror. A ray of light emanating from
this center will be reflected straight back to it. If the source of
light be moved a little nearer to the mirror, the light reflected will
be spread out more and come to a focus farther back from the glass; if
it be moved farther back from the glass, the rays will be focused nearer
the mirror. Thus if a light be placed at _A_, its rays will be focused
at _D_ and a light placed at _D_ will focus at _A_. This can be seen by
the lines which represent the rays of light. The two points at which a
source of light will thus focus are known as the _conjugate foci_ of the
mirror.
If such a mirror receives parallel rays of light, they will be reflected
and come to a focus at a point midway between the mirror and the center
of curvature. This point is known as the _principal focus_ of the
mirror, and the distance between it and the mirror is called the _focal
length_ of the mirror. A source of light placed at this point will throw
out parallel rays from the mirror. If the light be moved closer to the
mirror, the reflected rays will spread out; while if moved farther away,
the light will come to a focus at some distant point, as shown above.
Figure 74 can be used to illustrate the manner in which a concave mirror
reflects the light from an object placed before it. From the upper point
of the large arrow, rays of light emanate in all directions. All that
strike the face of the mirror are thrown to a certain point which can be
found by tracing out the lines, using the small arrows as guides. At
this point will appear the image of the top of the arrow. It will be
noted that it is inverted. The rays from the lower part of the arrow
are of course all reflected in the same manner.
[Illustration: FIGURE 74.]
With mirrors of this kind, the position of the object with reference to
the focal length and center of curvature is of great importance. If the
object be placed in the position shown as the image in Figure 74, the
image will appear as though it were in place of the object; it will be
much enlarged and also inverted. If the object is placed between the
principal focus and the mirror, it will appear to lie behind the mirror
as shown in Figure 75. In this case it will not be inverted.
[Illustration: FIGURE 75.]
When concave mirrors forming large sections of spheres are used, the
rays reflected from the outer edges will not all meet exactly at the
focal point. There will then be a somewhat fuzzy image formed. This is
illustrated in Figure 76. In order to obtain a perfectly clear and
distinct image, only the central part of concave mirrors should be used.
[Illustration: FIGURE 76.]
Convex mirrors are not much used. Sometimes glass spheres are set up to
show miniature reflections of scenery; convex mirrors are also found in
the lobbies of theaters and in places of amusement to amuse the patrons
with the caricatures of themselves reflected in them.
[Illustration: FIGURE 77.]
All of the rays that strike a convex mirror are reflected back in such a
manner that they seem to come from a common point behind the mirror.
This is shown in Figure 77. The center of curvature here is behind the
mirror but the paths of the various rays can be determined as before
explained. Thus we shall find that every ray, striking the mirror from a
certain point, is reflected back in a direction which gives it the
appearance of coming from a certain point behind the mirror. Two such
points are shown in Figure 77.
[Illustration: FIGURE 78.]
In Figure 78 we have drawn the arrow and the image it would produce in
the mirror. If the mirror forms a section of a sphere, the object
reflected will appear reduced in size in all directions. If the mirror
forms merely a section of a cylinder, a person standing in front of it
will appear very much shorter than natural but of full width thus
presenting a ridiculous appearance. Convex and concave mirrors are often
combined and if properly set, a person standing in front of one may see
himself either very much elongated or shortened.
CHAPTER XI.
REFRACTION.
If a straight stick or pencil be plunged into a vessel containing water,
it will appear to be bent. The reason for this appearance is given in
detail in Figure 79. The only light which can reach the eye from the
lower extremity of the stick must reach it by a path similar to that of
the bended ray at the left. According to this, the rays of light leaving
the bottom of the stick at _E_ bend at the water’s edge and meet the eye
as shown. To the eye the rays, by which it sees the end of the stick,
appear to come from the direction _F_; hence the stick is seen crooked.
When a ray of light passes from air into a denser medium such as water
or glass, the ray appears to be bent somewhat, as illustrated at the
left of Figure 79. This bending of the rays of light is called
_refraction_.
[Illustration: FIGURE 79.]
The fact of refraction can be further verified by placing an object _G_
into an empty vessel in the position shown. In this position the object
is not visible to the eye. By slowly filling the vessel with water, the
object will gradually appear and will seem to lie in the direction of
the straight dotted line.
[Illustration: FIGURE 80.]
The bending or refraction of light rays is governed by a law which holds
in all cases. The bending differs with different kinds of glass, with
water, with oils, or even with water mixed with chemicals; but no matter
how much the bending may differ with different materials, for each
material there is a fixed ratio between the angle at which the light
enters and the angle at which it continues in the material. This ratio
is known as the _index of refraction_. For water it is about 1.33; for
glass about 1.5; for the diamond about 2.5; etc. This can perhaps be
most easily explained by reference to Figure 80. Here the top of the
shaded space represents the surface of the glass at which the light
enters. The line _E_ is drawn at right angles to this surface and the
various lines, meeting in the center of the circle, represent different
rays of light falling upon the glass at different angles. The heavy line
shows the ray of light striking the glass and passing through it.
Similarly each line is drawn differently and the nature of the lines
will act as a guide by which the angle of incidence upon the glass and
the angle at which the rays continue through the glass can be traced.
When it is said that glass has an index of refraction of 1.5, or 3/2, it
means that the sine of the angle of incidence is 1.5 times as great as
the sine of the angle of refraction. The sine of the angle of incidence
is proportional to the length of a line drawn from the vertical _E_ to
the periphery of a circle, outside of the glass or other substance, to
where it meets the ray; and the sine of the angle of refraction is
proportional to the length of a line drawn from the same vertical to the
periphery of the same circle, inside of the glass or other substance,
where it meets the ray.
In order to lay out the path of a ray through the material, its angle of
incidence being known, we may resort to the construction shown in Figure
80. Draw the incident ray at its proper angle and the vertical line at
right angles to the surface of the material. From the point _A_, where
the ray enters the circle, and at right angles to the vertical line,
draw the line _1_. Measure the distance from _A_ to the vertical line
and divide it by the index of refraction; and on a continuation of the
line _1_, lay off the distance as found. From the point so located, draw
a line parallel to the vertical until it intersects the circle at the
bottom. From the point at which the ray enters the medium, draw another
line to where the last line drawn strikes the circle. This line will
give the direction in which the ray is propagated in the medium. For
glass, the index of refraction is taken as 3/2; hence the line _1_ must
contain three parts, while the continuation contains two of the same
length. Upon leaving the medium, the ray is bent back again so that it
continues in a direction parallel to that at which it entered, as shown
in Fig. 81; and thus the case is reversed.
[Illustration: FIGURE 81.]
[Illustration: FIGURE 82.]
Figure 82 is an illustration of how both refracted and reflected rays
may be several times reflected. If one will hold a match close to a
mirror, looking at it somewhat from the side, there will be visible six
or eight distinct reflections of it. These will be of different
intensity.
If a source of light has its origin in a medium denser than air, it will
be bent as shown in Figure 83. In such a case some of the rays do not
leave the medium at all but are reflected back into it, as indicated at
the right. The heavy line indicates what is commonly known as the
_critical angle_. This angle varies with different substances and is the
angle at which the refracted ray of light skims along the surface and
does not pass out. All rays emitted at a lower angle are returned back
into the medium. If an electric light were immersed in water, only the
rays of light at the left of the black line would be visible to an eye
outside.
[Illustration: FIGURE 83.]
Figure 84 is an illustration of an equilateral prism. Here we have
refraction on both surfaces and a light placed at _A_ would be seen by
the eye as located at _B_. With prisms it is easily possible to locate a
light so that it may be seen in two positions at the same time.
[Illustration: FIGURE 84.]
Figure 85 shows a double convex lens. Both sides of it are segments of
circles or sections of a sphere. The center of curvature or the radius
of one side is at _A_ and that of the other at _B_. Each minute particle
on the surface of such a part of a sphere may be looked upon as the
surface of a prism, the inclination of it being indicated by a line
drawn at right angles to a line passing from the point through the
center of curvature. This is illustrated in the figure by the short
heavy lines at right angles to the broken lines centering at _A_ and
_B_.
[Illustration: FIGURE 85.]
By using the construction explained with Figure 80 and using for the
left-hand surface the circle of broken lines and for the surface at _E_
the circle of solid lines, we find the path of the ray coming from _C_
to cross the principal optical axis at _A_. A similar construction for a
ray at the lower side of the lens would bring it to the same point and
furthermore all parallel rays would focus at this point. This point is
thus known as the _principal focus of the lens_, and the distance
between this point and the lens is called the _focal length of the
lens_. Every lens has two principal foci, one on either side and at
equal distances from the lens.
[Illustration: FIGURE 86.]
[Illustration: FIGURE 87.]
If, instead of subjecting the lens to parallel rays, we use rays
emanating from a central point on the optical axis, they will come to a
focus at some different point on the other side of the lens, as
illustrated in Figure 86. If the light be placed at the principal focus
of the lens, the rays leaving the lens will be parallel. If the light be
brought nearer the lens, the rays leaving it will spread out as shown in
Figure 87; they will leave the lens in a direction such as to make upon
an observer from in front the impression that they are coming from the
point _C_ behind the lens. If the light be placed beyond the principal
focus of the lens, the rays will converge to a point some distance ahead
of it. This distance varies with the position of the light and the two
points (light on one side and focus on the other) are known as
_conjugate foci of the lens_.
[Illustration: FIGURE 88.]
If an object be projected through a lens as, for instance, the arrow at
the right of Figure 88, it will appear upon a screen placed on the
opposite side, but will be inverted. The reason for this is that the
rays of light striking the lens from the top of the arrow will be
refracted as shown, cross the focal point _F_, and meet those which come
from the same point and pass through the center of the lens in a
straight line; and thus the image of the arrow head will appear at the
bottom; in a similar manner the image of the tail of the arrow will
appear at the top. The flatter the lens, the farther away will be the
point at which the image is formed.
[Illustration: FIGURE 89.]
If such a lens be placed over an object, the light will come to the eye
as shown in Figure 89. The solid lines show the rays by which the eye
receives the light and the broken lines show the direction from which it
appears to come. Thus we see the object much enlarged. A lens used in
this manner is spoken of as a _reading glass_, and the greater the
curvature, the greater its magnifying power.
[Illustration: FIGURE 90.]
[Illustration: FIGURE 91.]
In Figure 90 is illustrated a double concave lens. Parallel rays
entering this lens are scattered by it, as shown. Therefore, if such a
lens be placed over an object, the light from the extremity of the
object will come to the eye as indicated in Figure 91, but will give the
appearance of coming along the lines _H_ and _I_. Thus the object will
be seen much reduced in size. Such lenses are sometimes used by artists
to bring landscapes to a reduced size so that they may be viewed as a
whole more easily.
[Illustration: FIGURE 92.]
The general forms of lenses are shown in Figure 92,
(_a_) Is a plano-convex lens
(_b_) Is a double convex lens
(_c_) Is a convexo-concave lens, or convex meniscus
(_d_) Is a plano-concave lens
(_e_) Is a double concave lens
(_f_) Is a concavo-convex lens, or concave meniscus.
CHAPTER XII.
OPTICAL INSTRUMENTS.
In most optical instruments, lenses are used for the purpose of
gathering a large number of rays of light and altering the apparent
direction of the rays so that an enlarged picture may be presented to
the eye. In order to accomplish this, it is necessary that the rays of
light be bent or refracted. This refraction, we have already seen, is
always accompanied by a dispersion which causes the light to be
dissolved into its original colors more or less. This has been
illustrated by means of prisms.
[Illustration: FIGURE 93.]
[Illustration: FIGURE 94.]
Wherever a single lens is used, the light around the edges of the
illuminated space is always more or less colored, varying with the
illuminant used. Such coloring is most noticeable along the edges of
projected pictures but it also exists, though to a less noticeable
degree, over the whole field, showing least in the center.
The light which is thus refracted and dispersed by one prism may be
gathered again by another, as shown in Figure 93, but the light rays
after passing through the second prism will be exactly parallel to the
ray striking the first. The light coming out of the second prism will
appear white but it will be impossible either to enlarge or diminish the
size of a picture in this way; hence lenses, corrected to give white
light in this manner, would be of no use.
Fortunately it has been found that, with different kinds of glass, the
ratio of refraction to dispersion is different; and by combining two
pieces of glass of different nature, it is possible to recombine the
colors without causing the emergent ray to become parallel to the
incident ray. Consider, for instance, Figure 94 in which we have drawn a
prism made up of two different kinds of glass. If the right half were of
a glass having the same index of refraction as the left, for the red
rays for instance, these would continue through both in a straight line.
If the dispersion were less in the right half, i.e., if the violet rays
were refracted less--sufficiently less to cause them to approach the
red--they would meet the latter at some point outside of the prism and
combine into white light again, thus eliminating the colors ordinarily
visible through single glass lenses.
[Illustration: FIGURE 95.]
Whenever it is necessary to project especially good pictures upon a
screen, lenses corrected in some such manner as outlined above are
always used and the lenses are often combined as shown in Figure 95. In
this figure, _R_ indicates the line, through the principal axis, at
which the red rays refracted by lens 1 alone would strike; and _V_, the
line where the violet rays would be projected. The addition of lens 2
brings the red and violet together again at _W_. A combination of two
such lenses, placed the proper distance apart and the surfaces properly
proportioned, may be made to combine any two of the colors of the
spectrum. Hence even with these corrected lenses there is always some
coloring on the screen although it is hardly noticeable.
[Illustration: FIGURE 96.]
[Illustration: FIGURE 97.]
[Illustration: FIGURE 98.]
Figures 96, 97, and 98 are drawings showing the manner in which
objective lenses are usually made up. The types at the right and left
are used for camera work, while the one shown in the center is used
mostly for moving picture and stereopticon projection. The end having
the separate lenses is turned towards the light. Those shown in contact
are glued together by the use of Canadian balsam.
[Illustration: FIGURE 99.]
The optical system of the ordinary telescope is shown in Figure 99.
Light from the distant object _A_ is gathered by the large lens _B_ and
an image is formed as indicated by the small arrow. This image acts as
the object to lens _C_ and is projected to lens _D_ where the rays of
light are strongly refracted, entering the eye by angles which cause an
enlarged view of the object at _E_, as indicated. There must of course
be some means by which the lenses may be adjusted to each other for
focusing.
[Illustration: FIGURE 100.]
The arrangement of the opera glass in Figure 100 is quite different from
the above because of the reduced size of the instrument and for the
reason that an erect picture is desired, whereas the telescope above
gives an inverted one. The principal difference between the two is in
the eyepiece. In the opera glass this is a concave lens while in the
other it is a convex lens. In this case it is necessary to have the eye
very close to the lens to catch the rays of light. The opera glass, as
well as the telescope, must be provided with means of varying the
distance between the lenses according to the distance of the object
viewed, for the purpose of focusing. In some telescopes and also opera
glasses, prisms are used for the purpose of obtaining erect images.
Figure 101 will show how the rays of light entering a prism are
reflected and the image reversed thereby.
[Illustration: FIGURE 101.]
Figure 102 is an explanation of the reflecting stereoscope. Let the
black circles represent the eyes of the observer and let _M_ represent
two mirrors placed as shown. If two pictures taken by a stereoscopic
camera are placed as indicated by the arrows at the right and the left,
they will appear superimposed in the position of the arrow in the rear.
[Illustration: FIGURE 102.]
[Illustration: FIGURE 103.]
The refracting stereoscope is the one mostly used and the plan of it is
shown in Figure 103. Pictures for use with stereoscopes are taken by
special cameras provided with two lenses placed about as far apart as
the human eyes and mounted together. Stereoscopic effects may, however,
be produced even without this precaution and it is possible to obtain
some queer results by combining certain pictures.
In the so-called “Camera Lucida”, prisms of the type shown in Figures
104 and 105 are used. At the left is a combination lens and reflecting
prism which gives an erect image, and Figure 105 is a prism also
designed to give erect images. Such instruments are used for sketching.
They may be made to throw an image upon a small screen where its lines
may be traced out by the artist.
[Illustration: FIGURE 104.]
[Illustration: FIGURE 105.]
The most important optical instrument with which we have to deal is the
_projecting arc lamp_ and its optical system. The passage of light
through this system of lenses is altogether different from that passing
through a camera lens for instance. In a camera lens the picture is
formed upon the ground glass screen by the light reflected from the
object. From any single point of the object, rays of light strike all
parts of the lens, pass through it, and are recombined or focused at
some point behind it. Under these conditions, focus can be obtained only
at a certain distance behind the lens, this distance varying with the
distance of the object whose light is being received by the lens.
[Illustration: FIGURE 106.]
[Illustration: FIGURE 107.]
[Illustration: FIGURE 108.]
With our projecting lens we have no reflected light leaving the object
in all directions, but instead we have rays of light having definite
directions. This can be seen from Figure 106. The light used must come
from a point source, the smaller and the more intense it is the better.
This light is gathered by condensers, as shown at _C_, which are so
arranged as to focus the light in the center of the objective lens _D_.
In the moving-picture machine the light, before reaching the objective
lens, is passed through the film as indicated, the arrows representing
sections of film. The picture projected in this manner can be made to
appear upon a screen in front of the object lenses at any distance, but
the farther away it is, the larger it will be and the less bright the
illumination of it. A picture projected in this manner is always
inverted, and, in order to have it appear right side up, it must be
placed in position upside down. Figures 107 and 108 show arrangement of
lenses frequently used as condensers.
CHAPTER XIII.
OPTICAL ILLUSIONS.
[Illustration: FIGURE 109.]
[Illustration: FIGURE 110.]
[Illustration: FIGURE 111.]
[Illustration: FIGURE 112.]
The eye is easily deceived and is also very inaccurate in its judgment.
In Figures 109 to 111, all of the lines are of precisely the same
length, yet they appear to differ considerably. The reason for this
error in the estimation of objects is not known. It is especially
noticeable with such objects as a high hat; almost any one will estimate
the height of a silk hat as much greater than it really is. In general,
white objects also appear to be much larger than black objects. This can
be seen by the two inscribed squares one black and the other white in
Figure 112; both of these are of exactly the same size. Probably the
fact that more light reaches the eye from a light-colored object than
from a dark-colored one of equal proportions causes the impression of
greater size.
That the mind has the power of mixing contradictory or conflicting
impressions made upon it is proved by many facts. If a star, Figure 113,
be pinned to the center of a wheel and rapidly revolved, the center of
it will appear jet black; while the outer portions, made up of the
points and the white background, will appear grey which will gradually
fade to a lighter shade from the center black spot outward.
[Illustration: FIGURE 113.]
[Illustration: FIGURE 114.]
If a card be provided with a picture of a bird on one side and a ring on
the other, Figure 114, and this be rapidly spun on one corner, after the
manner of a top, the bird will appear to be in the center of the ring.
This effect is due to the persistence of vision; an image formed upon
the retina requires some time before it can be eliminated so that both
images appear together.
This persistence of vision and the power of suggestion are made use of
in a well-known act which consists of apparently throwing something, for
instance a guinea pig, into the air and causing it to vanish. To perform
this trick, the operator holds the pig in his hand and makes a few
motions suggestive of tossing it into the air. Then, with a final more
extreme motion on the downward swing of his hand, he drops the pig and
swiftly moves his empty hand upward. Due to the persistence of vision,
the audience actually still sees the pig and due to the suggestion of
tossing the eyes look upward, and thus the persistence of vision and the
power of suggestion create the illusion.
[Illustration: FIGURE 115.]
_Dissolving Views of Living Pictures._--This act requires a large glass
plate of good clear quality arranged upon the stage, as shown in Figure
115. _One_ of the poses is arranged as at the black circle behind the
glass and when illuminated is seen by the audience. The other is
arranged at one side. The lighting of both is connected to the same
dimmers in such a manner that when the light of one is increasing, that
of the other is diminishing. Thus the two figures are dissolved into one
another. The paths of the various rays of light to different parts of
the auditorium are shown in the drawing and it is possible to procure
perfect registry.
_Human Figure Floating or Performing in Air._--This act is arranged by
means of a large mirror placed upon the stage as shown in Figure 116.
In the _pit_ out of sight of the audience is a revolving table draped in
dull black and all of the pit is draped in the same manner. A figure
lying upon this table dressed in light clothes will appear erect in the
mirror as indicated by the arrow. If now the table is revolved, the
figure will be seen as turning over in the mirror. The figure upon the
table may perform a number of evolutions suggestive of floating, flying,
etc. The black cloth of the pit will reflect no light and only the
figure will be visible. If a glass plate is used instead of a mirror,
suitable scenery may be arranged back of it. The figure must of course
be brightly illuminated. By moving the table upon which the figure rests
across the pit, the figure will appear to move along.
[Illustration: FIGURE 116.]
_Head Suspended in Air._--This act is worked out in bright light. It
requires a mirror with a hole in the center of it large enough to admit
the head of a person, as shown in Figure 117. Above the mirror suitable
decorations are provided which cause one looking at the mirror to
imagine seeing the bac
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