|San Francisco Call, 08-May-1900|
RECENT SCIENTIFIC DISCOVERIES.
Copyright, 1900, by Seymour Eaton.
Note— These papers on Practical Science have been prepared for The Call's Home Study Circle by Professor William J. Hopkins of Drexel Institute.
VI. MOVING PICTURES.
If we look at one of the conventional pictures of a running horse— a picture twenty years old, preferably— and compare it with some of the more recent pictures of the same subject we may see a marked difference. In the older picture the horse is stretched out in an attitude which, to the eye not specially trained, suggests speed. In the more recent one he may perhaps be seen standing like an awkward statue on one leg, with no visible means of support for the rest of him. And yet this is. in a sense, true to life, for it represents accurately the position of the horse at one instant. Except to the technically trained eye it conveys no impression of rapid motion, for the eye does not see things that way.
Persistence of Vision.
The response of the optic nerve and its connections to any action which produces the sensation of light Is not instantaneous nor does the sensation cease at once with the cessation of the cause. It persists for an appreciable time, varying from about one-thirtieth to one-tenth of a second, according to the intensity of the light. In the case of the running horse, therefore, the rapidly changing positions blend into an impression which is pretty well represented by the kind of picture that was customary before the days of instantaneous photography.
It would never have occurred to any one before Muybridge had come to this country and had shown his instantaneous pictures of moving horses that any horse ever assumed such attitudes. Even with the evidence before their eyes the first sensation of the majority his hearers was one of amusement and incredulity. His photographs were obtained by using a series of cameras so placed that the horse came before each one in succession and caused the exposure to be made at the right instant by the breaking of a thread. With the cameras at the right intervals there was thus obtained a series of pictures which showed successive instantaneous attitudes of the horse, with brief breaks between. If these pictures could be presented before the eye, one at a time, in order and rapidly enough, the impression of one would persist until the next appeared and the horse would be seen running, trotting or cantering naturally. Such an instrument, very crude and simple, existed at that time and had been in use for many years as a scientific toy. It was the zoetrope, the forerunner of the kinetoscope and all its kin.
The zoetrope consists of an open cylinder, usually of cardboard, so mounted that it can be whirled on a vertical axis. In the upper half of this cylinder are cut equidistant vertical slits. The pictures in order showing the successive instantaneous positions In the motion to be represented are placed inside the cylinder, against the lower half, so that one picture is opposite each slit. On whirling the cylinder, therefore, keeping the eye at the row of silts, the pictures follow each other in rapid succession, only one being seen at a time. The sensation of each lasts until its image is replaced by that of the next, and the impression is that of the action of which the individual pictures show different stages.
Moving Picture Machines.
All machines for showing moving pictures by whatever name they may be called are nothing more than improvements and elaborations on the optical principle of the zoetrope. There are but few essential things which a successful machine of this sort must accomplish, but their accomplishment is not in every case altogether easy. The pictures must be presented so rapidly that there is no appreciable break between, either in light or in positions of the moving figures. The light must be admitted exactly as the picture is in place and must be cut off just before the picture Is changed. The different pictures must be shown in exact register or there will be a shifting or dancing effect which is not intended and is unpleasant.
Instead of a number of separate cameras to take the pictures, a single instrument is used, so arranged that the shutter is in practically continuous vibration at the rate of thirty or more exposures a second. In its proper place behind the shutter and lens a continuous strip of film is run in time with the shutter. Its motion is necessarily jerky, for it must be at rest for the exposure, while the shutter is open, and move ahead one space while the shutter is closed. By the development of this long strip of film there is obtained a series of negatives, from which positives are printed, in similar long strips, and these positive strips are used in the viewing or projecting instrument. If the positives are examined separately it will usually be difficult. If not impossible, to detect any difference in positions of the figures in pictures which lie near together in the strip, and when these pictures are passed through the projecting instrument at the same rate as that of the taking camera the movements of the figures upon the screen appear natural and lifelike.
The projecting instrument for moving pictures is a regular projecting lantern, with the addition of mechanism, attached to the objective, for keeping the shutter and the pictures in motion. This motion is kept up by a motor or by a hand-wheel, and the shutter or fan is run by the same mechanism that moves the film. A picture is held against the opening while the lens is uncovered, then released as the vane covers the lens, and the next one takes Its place, to be shown, in turn, the instant the vane has passed.
The chief use to which moving pictures have' been applied thus far is that of entertainment, and unfortunately the subjects which have aroused the most interest have been prize-fights. There are, however, many directions in which this instrument may prove of great value. It is proposed to make use of it in war and how far such an idea can be carried out we shall perhaps know better after the close of the present war in Africa. There are certain practical difficulties in having a kinetoscope camera at the front always ready for service, even If its use were permitted.
The value of a series of instantaneous photographs taken at short intervals in analyzing rapid motion is sufficiently obvious. By passing the pictures through the viewing instrument at a reduced speed the motion may be made as slow as we wish and its nature clearly seen. Another application of this principle which has recently been proposed is less obvious. It is proposed to take photographs at long intervals of movements which are very slow. Then, passing the series of_ pictures from these negatives through the viewing instrument at the usual speed the slow motion becomes rapid. Suppose, for example, that a photograph is taken every few hours of a sprouting seed and growing plant. When shown by the viewing or projecting instrument the sprout may be seen breaking through the ground, gaining in height and size, putting forth leaves, buds and branches and reaching its maturity, all in a few minutes. The same method may be applied to many things other than plants. To this modification of the principle of moving pictures it has been considered necessary to give a new name -- the "phantoscope."