The Cine Technician (1939)

46 T II E C I X E-TEC HNICIA X June-July, i same spol in the eye. The observer thus gets an impn ;ion of continuous presence. This taking place ill along the path of the moving object naturally causes it in appearance to fill the whole space. Fortunately as this, like most other experimental facts, admits of simple verbal expression, one sentence suffices — we continue to experience the visual effect of light after it has ceased to act. Such is this phenomenon known as Persistence of Vision. There is a deficiency of the human i ye of which we have taken advantage. This wonderful organ of ours has a defect which is known as "visual persistence." The brain persists in seeing an object after it is no longer visible to the eye. Let us get it clearer. The eye is a wonderful "camera." The imprint of an object is received upon a nervous membrane which is called the retina. This is connected with the brain, where the actual conception of the impression is formed, by the optic nerve. The picture, therefore, is photographed in the eve and transmitted from that point to the brain. Now a certain amount of time must elapse in the conveyance of this picture from the retina along the optic nerve to the brain, in the same manner that an electric current flowing through a wire, or water passing through a pipe, must take a certain amount of time to travel from one point to another, although the movement may be so rapid that the time occupied on the journey is reduced to an infinitesimal point and might be considered instantaneous. When the picture reaches the brain, a further length of time is required to bring about its construction, for the brain is something like a photographic plate, and the picture requires developing. In this respect the brain is somewhat sluggish, for when it has formulated the picture imprinted on the eye, it will retain that picture even after the reality has disappeared from sight. This peculiarity can be tested very easily. Suppose the eye is focussed upon a white screen — a picture suddenly appears. The image is reflected upon the retina of the eye, and transmitted thence to the brain along the optic nerve. Before the impression reaches the brain, the picture has vanished from the sight of the eye. Yet the image still lingers in the brain ; the latter persists in seeing what is no longer apparent to the eye, just as plainly and distinctly .1 il it were in full view. When the image does disappear it fades away gradually from the brain. True, the duration of this continued impression in the brain is very brief. In the average person it approximates about 1 10th to 1 24th of a second, subject to the degree of intensity, duration, and colour of the light received by the eye. Still, in a fraction of time a good deal may happen, and in the case of animated photography it suffices to bring a second picture before the eye ere the impression of the preceding image has faded from the brain. It is interesting to note that the first written reference i" Persistence of Vision is contained in the fourth book of "De Rerum Natura," by Lucretius, dated about 65 B.C. He says, "This (perception ol movement) is to be explained in the following way, that when the first image passes off and a second is afterwards produced m anothei position, the former then mviux to have changed its gesture. This we must conceive to be done by a very rapid process " This, ol course, only expresses the fad oi persistent vision, and mentions no means lor its demonstration. Throughout the ages many men put forward theories with regard to persistence of vision. This history needs a further at tide to give it full justice, and perhaps I may have the opportunitv of referring to it at some later date, so we will take up our story again from the time when inventors were trying to reproduce movement by photographic means. With the advent of snapshot photography, a Mr. Muybridge, an Englishman, resident in San Francisco, in the year 1872 conceived a novel idea for securing a series of snapshot pictures in rapid succession. He built a studio beside a track, in which 24 cameras were placed side by side in a row. On the opposite side of the track, facing the studio, he erected a high fence painted white, while across the track between the studio and the fence, 24 threads were stretched, each of which was connected with a spring which held in position the shutter of the camera. When all was ready a horse was driven over this length of track at a canter, trot or walk, as desired, and as the animal passed each camera, it broke the thread controlling its shutter, so that the horse photographed itself in its progress. After this method of photographing the movements of a horse, many men attempted taking pictures on glass plates. \mong them was William Friese-Greene, who in 1880 engaged himself in scientific research relating to photography, particularly with regard to photographing animated objects in motion. In 1885, he built an adapted camera which was used for taking "Motion Pictures" by merelv turning the handle of the machine, this operating a circular shutter to permit the exposure of separate sections of a sensitised glass plate. In 1887 he devised and constructed a camera to take photographs upon a sensitised strip of paper, about 50 feet in length, which was wound upon two rollers, a feed roller and a take-up roller, which strip of paper passed through a guide across the light aperture of the camera, the strip being intermittently exposed upon the revolution of the circular shutter, when the apertures therein registered with the light aperture of the lens. In this camera he employed a strip of paper perforated at both edges, the perforations being for the purpose of permitting registration of a pair of sprocket wheels with either edge. These sprocket wheels were worked by what was termed a star movement, to give the intermittencv of motion. Turning the crank of the camera caused the star movement intermittently to engage a toothed wheel fixed to the shaft carrying the sprocket wheels, which in turn, when operated by the star movement, moved the film forward step by step across the light aperture, permitting successive portions of it of equal area to be uniformly and regularly exposed while stationary, thus producing upon the strip of paper a series of equally spaced equidistant images or negatives, each negative showing the animated object in its successive phase of motion. The pair of sprocket wheels were mounted on a common shaft and engaged the perforation in the edges of the paper strip at a point intermediate between the feed and take-up rolls and just below the aperture, drawing the strip downward across it. The feed roll by the series of pulls of the sprocket wheels upon the strip was caused to rotate in a successive series of motions, each causing to unwind therefrom sufficient of the paper to produce a loop or slackness of the Strip between the feed roll and the guide, so th.it in the taking of pictures and turning the handle of the camera, the paper strip should not be under such stress or strain arising from the pulling ol the sprocket wheels as to cause a breakage oi tearing. {Continued at fool of next page)