It has been pointed out in a previous chapter that cinematography is nothing more or less than an optical illusion. Further proof of this assertion exists in plenty. When following the projection of a picture upon the screen, one is often perplexed by a curious effect, or a movement which appears to be in opposition to all the known laws of motion. This happens not only in trick work where such odd and startling effects are introduced purposely, but in straightforward every-day topical subjects.

For instance, it must have been noticed that when a ship or railway train is in rapid movement, and is photographed from a fixed stationary point, such as the quay or platform, the moving object appears to stand out in bold relief against the background. One gathers a very comprehensive idea of its length, width, height, and the comparative size of all its integral parts, such as the guns on the ship's deck or the locomotive's cylinders. It is a curious stereoscopic effect, but at the same time is not truly so, because it is only the moving object which appears to possess solidity. The foreground and background remain as plane surfaces so that it is impossible to obtain an idea of distance. This effect arises from the fact that what might be described as the central part of the picture is moving or continually changing, thereby compelling all the objects attached to its length and breadth to assume relief in regard to the other parts of the picture.

But if the camera with which the pictures are taken is placed upon the moving object itself, then the whole of the resulting picture stands out in a truly stereoscopic manner. One gathers an impression of distance between the various objects on the screen. Everything is shown with form and solidity in precisely the same way as if one were looking through a hand stereoscope upon a photograph taken stereoscopically. This effect is due to the fact that all the planes are moving continually.

But probably the most bewildering puzzle is the moving wheel. A carriage or waggon is seen advancing across the screen from left to right, but the spokes of the wheels, on the other hand, seem to be moving in the opposite direction. At other times the spokes move in successive spasmodic jumps, or appear to be stationary, so that a curious skidding effect is produced, notwithstanding that the rim itself is seen to be revolving normally.

There have been many explanations of this extraordinary effect, and in one instance the higher mathematics were pressed into service without any great success. The most convincing explanation known to the writer is that given him by Monsieur Lucien Bull, the assistant-director of the Marey Institute, where phenomena of this class are minutely investigated, because they accord with the work of that unique and admirable institution. By Monsieur Bull the illusion was explained very easily, but, curiously enough, in carrying out the experiments to this end, he encountered another illusion equally strange.

To reduce the explanation to its simplest form we will suppose that a wheel has four spokes spaced equidistantly, that is, 90 degrees apart, and that the wheel is moving from right to left. As a matter of fact such an example is not the best for the purpose, but it shall be taken merely because it is the simplest to understand. An exposure is made, the wheel being photographed in the position shown in Fig. 3. The lens is eclipsed by the shutter, and the film is jerked downwards into position in the gate so as to bring a fresh unexposed surface before the lens. While this operation is taking place, we will suppose that the wheel, continuing its forward movement, completes one quarter of a revolution. Consequently when the second exposure is made spoke 1 has moved 90 degrees, which is the angle between each spoke. Accordingly it now occupies exactly the same position as that of spoke 2 at the time of the first exposure. Spoke 2 has moved to the position formerly occupied by spoke 3. Spoke 3 has travelled sufficiently to take the place of spoke 4, while 4 has gone to that of 1 (Fig. 4). If four exposures are made, and the spokes move 90 degrees each time the lens is closed, when the four pictures are thrown successively upon the screen they will look exactly alike. The spokes will appear to be quite stationary, although the rim of the wheel will have moved a distance equal to its circumference across the screen. Consequently, if a dozen, a hundred, or a thousand exposures are made under these conditions, the spokes moving 90 degrees between each exposure, a quaint skidding effect will be produced. All the spokes being alike the eye is unable to detect that any displacement has taken place between one exposure and another. This impression of the spokes standing still while the wheel is moving, must arise in every case in which the wheel moves sufficiently to cause the spokes to cover a distance equal to the angle between them during the interval while the lens is eclipsed by the shutter. It will happen equally whether the wheel has four, sixteen, or more spokes.

Now we will suppose that the revolving speed of the wheel is retarded, causing less than a quarter of a revolution to be completed between each exposure. The spokes, let us say, move through an angle of 85 degrees instead of 90 degrees while the lens is eclipsed. The eye at first receives the impression shown in Fig. 3. As the wheel only covers 85 degrees during the eclipse, in the second picture the eye observes that movement has occurred. Spoke 1 is now behind the point formerly occupied by spoke 2 (shown by the dotted line in Fig. 5) in the first exposure. The lens is eclipsed once more, and the spoke moves another 85 degrees. When the next picture is seen spoke 1 has fallen still farther behind the 90 degrees mark, and this indication of less movement than the right angle becomes accentuated with each succeeding exposure. Accordingly, the spokes in the successive pictures appear to be moving at a less speed than the rim of the wheel, and forthwith the eye imagines that the spokes are travelling backwards, although meantime the wheel rim is seen to be advancing across the screen. This remarkable effect is produced whenever the advance of the wheel is such as to cause the spokes to move less than the angle between them, no matter what the size of the angle may be.

We will now suppose that the revolving speed of the wheel is accelerated so as to cause more than a quarter of a revolution to be made while the lens is eclipsed—that the spokes move forward 95 degrees between each exposure. In this case, while the first picture will show the position indicated in Fig. 3, the next exposure will show spoke 1 in the position shown in Fig. 6, that is, in advance of the angle of 90 degrees and in advance of the position occupied by spoke 2—(see the dotted line)—in the first exposure. In the third picture the spoke will be shown still farther in advance of the right angle mark, and the effect will be produced of the spokes apparently gaining upon one another. When a series of pictures taken under such conditions is thrown upon the screen in rapid succession, the spokes and rim will be seen to be moving harmoniously in the forward and correct direction. Accordingly natural movement of the wheel only can be shown when the spokes of the wheel, irrespective of their number, move a distance equal to more than the angle between them.

In the course of elucidating this problem Monsieur Bull discovered another curious optical illusion produced by the moving wheel. Still taking the four-spoke wheel as an illustration, we will suppose that between each exposure the spokes are displaced a little more or a little less than half the angle between them. As the spokes are set 90 degrees apart, the half-way point will be 45 degrees. When a succession of such pictures is thrown upon the screen, it is not four spokes which are seen, but eight (Fig. 7). Monsieur Bull is engaged upon a series of experiments to ascertain why this peculiar optical illusion should prevail, and the explanation will prove interesting.

Another interesting and more conclusive illustration of the optically illusory properties of the cinematograph was demonstrated to me by Monsieur Bull. In order to be absolutely positive that an apparatus which he uses in certain cinematographic investigations should maintain the speed he desires, he has contrived a tuning-fork control for his electric motor. This tuning-fork resembles a large trembler blade, such as is used in the high-tension accumulator and coil ignition system upon motor cars. This particular instrument is timed to make, say, 40 vibrations per second, and at this speed, of course, it emits a distinctive musical note. This tuning-fork controls the electric motor driving the apparatus. For the purpose of illustration we will suppose it to be necessary that the speed of the motor shall not exceed 40 revolutions per second. In the earliest experiments he depended upon his ear to detect whether the motor and tuning-fork were in synchrony. He varied the speed of the motor until its hum was dead in tune with that of the tuning-fork.

But, as he thought that his ear might not be infallible, he devised an ingenious synchronising apparatus based upon the cinematographic principle. A small disk of cardboard provided with two holes near its edge, at opposite points of the circumference, is mounted upon the spindle of the tiny motor. Behind this disk is placed a small adjustable mirror. A pencil of electric light is projected horizontally in such a manner that it strikes the cardboard disk at right angles, and, when a hole on the disk is brought into line with it, it passes through and falls upon the mirror. The mirror is then set so as to reflect and focus the pencil of light in a small circle upon the free vibrating extremity of the tuning-fork. Naturally a strong shadow is thrown by the latter upon the white wall behind.

In the daylight the vibration of this fork is distinctly visible, and although it is slight and rapid it can be followed without any effort. But when the room is darkened, the ray of light is thrown upon the tuning-fork from the mirror. When the motor bearing the cardboard disk is set in motion a very curious effect is produced. The pencil of light reflected against the tuning-fork becomes interrupted twice in every revolution of the disk, that is 80 times per second, so that, looking at the background upon which the tuning-fork is silhouetted, the effect produced is precisely similar to that observable upon the cinematograph screen, where the passage of the light from the lantern is interrupted by the rotary action of the shutter. If the revolving speed of the motor, that is the number of revolutions per second, is the same as the number of vibrations per second of the tuning-fork, viz. 40, the end of the fork, as one looks at the illuminated circle on the wall against which the shadow is thrown, appears to be at rest. One only needs to touch the end of the fork, however, to be certain that it is vibrating.

Now if the motor be thrown out of synchrony with the tuning-fork, even if it makes only 39 or 41 instead of 40 revolutions per second, the disturbance is shown instantly, because looking at the illuminated tuning-fork one observes it jumping spasmodically. This movement becomes more pronounced as the harmony between the revolutions of the motor and the fork is disturbed, the jumps of the blade at times being apparently of a very severe character. Moreover, curiously enough, under the illumination of the ray of light the erratic movements of the blade appear to be three or four times more severe than they really are. But as the motor revolutions and the tuning-fork vibrations are brought into synchrony, the movements grow quieter, until at last the tuning-fork once more appears to be quiescent.

The explanation of this quasi-cinematographic illusion, which is as interesting and as puzzling as that of the wheel, is very simple, for it is based indeed upon the same phenomena. As the cardboard disk is provided with two small holes spaced 180 degrees apart, the passage of the ray of light is intercepted by the opaque section of the disk 80 times per second when the motor revolutions and the tuning-fork vibrations are in absolute synchrony. The result is that at this speed the light strikes the tuning-fork each time at the instant it is at the half-way point in its oscillating travel. One hole in the disk comes before the light when the blade has completed half its movement in one direction, while the second hole comes into line with the light when the blade is at the same point on its return journey. Consequently the light falls upon the blade at the same spot every time, causing the eye to imagine that it sees the blade always in the one position as if under a steady ray of continuous light. Hence comes its apparent quiescence. But directly the speed of the motor is altered in relation to the vibration of the tuning-fork, the rays of light catch the blade at varying points in its travel, and these changes, coming in quick succession, convey the visual idea of movement. Acceleration of the motor so that its revolving speed per second exceeds the number of the tuning-fork vibrations, causes the perceptible movements to be made more quickly, while on the other hand deceleration slows them down. In reality the eye imagines that it sees more than what actually takes place; it imagines that the blade of the fork is kicking spasmodically and viciously, whereas in fact the extent of the movement to and fro is constant and never changes.

While the experiment is peculiarly fascinating, its application is extremely useful to the worker. It offers a means of being absolutely certain about the speed at which the instrument utilised in a particular investigation is running, so that the resulting calculations may be completed without the slightest error.