Please see the [Transcriber’s Notes] at the end of this text.

Books by
Horstmann & Tousley

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 Con-
struction Tables,” “Practical Armature and Magnet
Winding,” “Electricians’ Operating and Test-
ing Manual,” “Modern Illumination.”

ILLUSTRATED

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

MOTION PICTURE OPERATION
STAGE ELECTRICS AND
ILLUSIONS

CHAPTER I.
THE ELECTRICAL CIRCUIT AND ELECTRICAL HAZARDS.

Two and Three-Wire Systems.

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.

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.

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.

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.

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.

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.

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.

FIGURE 3.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.

FIGURE 5.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.

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.

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.

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.

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.

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].)

FIGURE 8. FIGURE 9. FIGURE 10.

FIGURE 11. FIGURE 12. 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.

FIGURE 14.

[Figure 14] shows a form of McIntosh stereopticon lamp.

FIGURE 15.

[Figure 15] is a Kliegl lamp for open arc lamps.

FIGURE 16.

[Figure 16] is an Edison lamp used for motion picture work.

FIGURE 17.

[Figure 17] is a Kliegl lamp used for focusing purposes.

FIGURE 18.

[Figure 18] shows the Powers lamp.

FIGURE 19.

[Figure 19] shows one of the Motiograph Company lamps.

Optical System.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Example: With a lens of 5¹⁄₂ 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.

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.

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.

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.

FIGURE 26.

The diameter of the ordinary condenser lens is 4¹⁄₂ 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 6¹⁄₂ 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 7¹⁄₂-inch and two 6¹⁄₂-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.

FIGURE 27.

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.

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.

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.