For complete success in moving-picture work it is essential to have an elementary knowledge of the principles upon which the art is based. Although pictures are said to be shown in motion upon the screen, no action is reproduced as a matter of fact. The eye imagines that it sees movement. Each picture is an isolated snap-shot taken in the fraction of a second. In projection upon the screen, however, the images follow so rapidly one after the other and each remains in sight for so brief a period that the successive views dissolve into one another. The missing parts of the motion—the parts lost while the lens is closed between the taking of each two pictures—are not detected by the eye. The latter imagines that it sees the whole of the process of displacement in the moving objects. In fact it sees only one-half—the half that occurred in those fractions of seconds during which the lens was open. What occurred while the lens was shut is not recorded. Animated photography, therefore, is an optical illusion purely and simply.

The fact that an appearance of natural movement is seen under these conditions is due to a physiological phenomenon which, for the want of a better explanation, is termed "persistence of vision." This peculiarity of the eye and brain remains a scientific puzzle, and although in one or two quarters the theory of visual persistence is ridiculed, the iconoclasts have not yet brought conclusive testimony to upset it. The whole subject of persistence of vision in its relation to moving-pictures is discussed at length by the present writer in a former book to which he would refer such readers as may wish for information on this subject.[1]

The eye is about one million times faster than the most rapid sensitized emulsion which chemists have yet produced. So there is nothing wrong about the popular opinion that the organ of sight is the quickest of the senses. Yet it is not so quick that it cannot be deceived. If the pictures of a cinematograph are projected upon the screen at the rate of so many per second, the effect upon the eye is that of perfectly natural movement. The laws that govern this illusion have been discovered in a very interesting way. A positive film was prepared, but between each successive image a wide white line was inscribed. This film was then passed through the projector, and the pictures were thrown upon the screen at the speed generally accepted as being necessary to convey the effect of natural movement; but animation could not be produced at all, however rapidly the pictures were projected. The reason was simple. Immediately after a picture disappeared from the screen the white flash occurred, and notwithstanding its instantaneous character it was sufficient to wipe out the image of the picture, which without the white line would have lingered in the brain. Even when the pictures were run through the projector at thirty per second, no impression of rhythmic movement was obtained; they appeared in the form of still-life pictures with spasmodic jumps from one to the other. They failed to blend or dissolve in the brain, notwithstanding that the white flash in some cases was only about one ten thousandth part of a second in duration.

Another film of the same subject then was passed through the projector under conditions exactly similar except that the line dividing the pictures in this case was black instead of white. When this picture was thrown upon the screen, animation became apparent directly the speed attained sixteen pictures per second, because after one image had vanished from the screen it persisted in the brain, in spite of the black flash, until the next picture appeared. Thus, the requisite dissolving effect was obtained. The black flash did indeed produce a defect like that which was common in the early days of cinematography and was characterised generally as "flicker." But it did not suffice to ruin the illusion of movement. A white flash destroys apparent motion, owing to the brain being extremely sensitive to white: a black flash of equal duration exercises no ill effects.

In the latest development of the art, one inventor has taken advantage of this peculiarity. He has perfected a practical system wherewith the shutter of the camera may be abandoned because each picture is cut off from its neighbour by a very thin black line. An improved mechanism jerks each picture off and brings the next one on the screen very sharply, so that an effect is produced like that obtainable with the shutter and without any impression of flicker. It may be pointed out that with this invention there are none of the aberrations described in a later chapter, such as the spokes of a wheel appearing to move in the reverse direction to which the rim is travelling.

The next question is that of the speed at which it is necessary to take and to project the pictures in order to get an apparently true impression of natural movement. This factor to-day is governed almost entirely by commercial considerations. It has been found, as a result of elaborate investigation, that a speed of twelve to sixteen pictures per second is the minimum wherewith in monochrome pictures animation is obtainable. But this applies only to general work, such as records of ordinary scenes, topical events and stage plays, where the action of the moving objects is comparatively slow. In these instances an average of sixteen pictures per second in photographing and projecting gives completely satisfactory effects.

But in reality the speed is a variable quantity: it must be adapted to the subject and the character of the work in hand. In other words, strictly speaking, the speed must be accommodated to the velocity of the subject so far as photographing is concerned, and also, in a lesser degree, to the distance of the moving object from the lens. For instance, when a man, walking four miles an hour, is photographed at sixteen pictures per second, the movements recorded are far from being natural or rhythmic. On the screen he appears to walk with a disjointed action. To obtain a lifelike result, his pace should be slowed down 75 per cent., or the photographing speed should be accelerated to seventy pictures per second at the least. This fact is illustrated very conclusively in pictures of soldiers marching: they appear to advance like automatons. Again, in photographing animals, a complete movement is often lost between successive pictures. A cat in one picture will be seen to the right; in the next picture it is on the left, having sprung from one side to the other during the brief interval the lens was closed. When extremely rapid movements have to be recorded, the photographing speed has to be accelerated to an extreme degree, up to ten thousand pictures or more per second in the case of a bullet leaving the muzzle of a rifle, and up to two thousand pictures per second to catch the movements of a dragonfly's wings. On the other hand, in photographing very slow movements like the growth of a plant, one picture per hour may be adequate.

In projection the speed can be adjusted. The ten thousand pictures per second may be decelerated to sixteen per second to allow the movement to be followed, and although the rifle bullet may appear to crawl through the air, the movement is perfectly correct. Similarly the very slow motions must be accelerated to sixteen pictures per second to obtain evident animation. These two extreme phases of cinematographic investigation are described at length in another part of this volume, but are mentioned here merely to show that the photographing speed is a somewhat elastic factor, to be adapted to circumstances in order to produce passably natural effects.

For everyday work, however, a speed of sixteen pictures per second is sufficient and represents the generally practised velocity. Possibly in the near future the speed will be accelerated to twenty, twenty-two, or twenty-four pictures per second, as the present speed is generally admitted to be too slow. The eyes of the regular picture palace patrons have become trained, as it were, with the result that there is an appreciable strain of the eyes, while the disjointed character of the movements on the screen may be detected. But when the taking and projecting speed is accelerated by 50 per cent. the picture stands steadier upon the screen, the movements are more natural, and there is an entire absence of that automaton effect which is so characteristic of most pictures taken under prevailing conditions. These considerations do not affect photo-plays produced in the studio so materially, because there the actions of the players can be slowed down to suit the conditions.

One of the leading manufacturers is earnestly considering the advisability of accelerating the taking and projecting speeds up to about twenty pictures per second, and private investigations and experiments have certainly demonstrated the value of such an improvement. Unfortunately two difficulties prevent its immediate realisation. An increase of only four pictures per second represents an increase of 25 per cent. in the consumption of the film, and therefore in its cost. The other difficulty is more serious. Existing apparatus, both cameras and projectors, are geared to eight pictures per turn of the handle. This involves two complete revolutions per second. Consequently the gearing of the mechanism would have to be altered, and this is a more troublesome question than appears at first sight. Some time may elapse before a forward step is taken in this direction. In matters of this character the cinematograph industry is notoriously conservative, although the moment one firm courageously adopts an accelerated speed, the higher quality of the resulting pictures will force the others to follow the example.

As a matter of fact the decision to adopt sixteen pictures per second was taken somewhat haphazardly without any scientific investigation. When it became standardised, film was expensive. Accordingly, efforts were made to secure the requisite effect with the minimum expenditure of film. Machines were built to coincide with these requirements, and the original designs have been followed slavishly in their broad outlines ever since.