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.
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
FIGURE 28.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
FIGURE 34. FIGURE 35. 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.
L—Intermittent Sprocket.
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.
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.