THE combination of the illusion of movement given by the motion picture with colors has been for many years the hope of numerous inventors. But a brief sketch of their work can be given, as it would require a treatise larger than this book to exhaust the subject.

The fundamental basis of the motion picture is the phenomenon of persistence of vision. A bright light produces on the retina a certain impression, which lasts as long as the light is burning and a little longer, that is to say, the sensation of light is not instantaneously wiped out. If a succession of pictures be rapidly projected, about sixteen per second, the impression of the first picture persists on the retina until the second one is also there and is superimposed, and the same with succeeding pictures; therefore we have the impression of a complete picture, the sum of the individual phases of a movement, etc.

Precisely the same thing occurs with colors, The rapid alternation of red and green gives us the impression of yellow, of the constituents of which we are no longer conscious; so if a succession of red, green and blue pictures are rapidly and alternately presented to the retina, the composite result will be a moving picture in colors. The first suggestion as to such a process was made by H. Isen-see, a German, in 1897, although Cros in 1867 had suggested the synthesis of a single picture by this method.

Isensee proposed to take the pictures through the tricolor filters and project them in the same way. Several inventions were patented in which the same idea was involved; but the great disadvantage of this process, and in fact of all processes in which different phases of a movement are represented in different pictures, is that one cannot obtain a perfect composite result. This is clearly seen if one imagines the use of the three-color filters with successive exposures of a very simple object; assume that a man is holding his arm at right angles to his shoulder and drops it to his side, and that during the movement of the arm we take three pictures. It is obvious that we have a red picture of the arm in one position, a green picture of the arm in another position and a blue picture in another. It is impossible to register these, and they will not superimpose sufficiently well on the retina to be sharp; we shall see the arm first with a red fringe at the top, then with a green fringe at the top and a blue fringe at the bottom and so on, so that a sharp single picture by this method is impossible.

Another and very serious defect is technically known as color bombardment. This is the physical effect produced by the rapid alternation of the colors. With some people this produces intense discomfort and severe frontal headache. It is probably due to the imperfect achromatism of the eye, for while the main effect on the screen is a homogeneous color, yellow, for instance, in the alternation of red and green, yet there is a peculiar pulsation or throbbing which is not flicker and yet is closely allied to it.

These troubles led to attempts to take and project the three constituent pictures at once, so that each picture was complete in itself as regards color. While this obviated the color bombardment trouble, it introduced other troubles in the necessity for special optical devices, which not only are costly but in the use of which one is much hampered by the fact that the average cinematographic lens ranges from two to three inches in focal length, and, therefore, one is cramped for room. If more than one lens be used, that is, either two or three vertically or horizontally disposed, the element of parallax is introduced, which is also sometimes called stereoscopic parallax. Exactly the same phenomenon appears in our own vision; each eye sees a slightly different view of an object and all objects beyond the particular plane focused on appear double and wanting in sharpness. This can easily be seen by holding a pencil or some such object at about ten inches from the eyes and focusing them on it, the background being a window with cross bars about a yard away. When the pencil is sharply focused the window bars will be found to be double, and if the bars are focused the pencil will appear as two. We are not conscious of this ordinarily, as the eye automatically focuses itself and ranges over the whole field of vision, so that we obtain the sum of the individual impressions. In the case of the cinematographic camera, there is no variation of focus, so that parallax or the doubling of near or distant objects is distinctly seen.

Another trouble which is purely a mechanical one is that the normal rate of taking pictures, and exhibiting them, is sixteen per second. If the single objective be used, the film has to travel three times as fast as for black and white. If three juxtaposed lenses be used vertically, one has to make the film travel three picture spaces at each exposure, and if they are horizontally juxtaposed, then triple width film must be used.

These difficulties led G. A. Smith in 1907 to revert to a two-color process, in which alternating pictures were taken and projected through a rotating shutter with red and green sectors. This process was known as Kinemacolor and some excellent results were shown. But this process was handicapped by color bombardment. It was simpler than those processes in which simultaneous projection of the constituent color pictures is used, as the only extra fitting for the projector was the rotating shutter, and possibly a few gear wheels to speed up the travel of the film. It may be accepted as an axiom that any process that requires a special projector or special fittings is hopelessly out of the commercial running.