Fig. 250.
Fig. 251.
A still more ingenious application of this principle we owe to Plateau, who described it in 1833, under the title of the phenakistiscope; and also to Stampfer, who independently devised the same arrangement about the same time, and named it the stroboscopic disc. The reader may, at almost any toy-shop, purchase one of them, provided with a number of amusing figures; or he may easily construct for himself one which will exemplify the principle. He requires no other materials than a piece of cardboard, and his only tools may be a sharp penknife, a pair of compasses, and a flat ruler. Let him draw on his cardboard a circle of 8 in. diameter, and divide its circumference by eight equidistant points. From these radii should be drawn with the point of the compasses, and equal distances from the centre marked off upon them, to fix the centres of the small circles, which must all have exactly the same size (say, 1 in. in diameter) and be marked by a distinct line. In these are to be marked the hand of a clock-face in the positions shown in Fig. [249]; and finally, in the direction of the radii, narrow slips are to be cut out of the cardboard as shown. If a pin be put through the centre of the disc, attaching it thus to the flat end of a cork, so that it can freely rotate in its own plane, and the disc be turned rapidly round, as in Fig. [250], in front of a looking-glass, while the spectator looks through the slits, he will see the hand on the little dial apparently turning round, with rather a jerky movement it is true, somewhat like the dead-beat seconds-hand that is sometimes seen on clocks. The illusion is best when the slits are so narrow that only one of the several images is visible by reflection, namely, that which is adjacent to the slit. Thus, as the disc rotates, each little circle is visible for an instant as the slit passes in front of the spectator’s eye; and if the rotation be sufficiently rapid, the impression of the disc is permanent, as it is constantly being renewed by the successive circles, while, on the contrary, the hands, having different positions, produce images in different positions, giving the appearance of a jerky rotation. The instruments sold in the shops have sometimes a thin metallic disc with the slits in it, and a series of designs printed in smaller paper discs. The paper discs may be screwed on the other disc as required, and a button on a pulley with an endless band is provided for producing the rotation more conveniently. Fig. [251] shows one of the pictures for a disc with twelve slits, and the effect produced by it is that of a dancing figure.
Another arrangement for showing the same illusion has lately become a very popular toy, and quite deservedly so, for it has the advantages of requiring no looking-glass, and of making the effect visible to a number of persons at the same time. This apparatus, which has been termed the Zoetrope, consists simply of a cylindrical box, like a drum with the upper end cut off. It is mounted on a pivot, which permits its revolving rapidly about its vertical axis when touched by the finger. The cylinder has a number of equidistant vertical slits round the upper part of its circumference. The figures which produce the illusion are printed on a slip of paper, which is placed in the lower part of the drum, and when this is in rapid rotation, and the figures are viewed through the slits, the illusion is produced in exactly the same manner as in the revolving disc.
Fig. 251a.—Edison’s Kinetographic Theatre.
At the end of the article on the phonograph in a subsequent page, the reader will find a remark as to the effect that might be produced by a combination of that instrument with instantaneous and simultaneous photographs of some famous speaker. This combination has now been accomplished by the great inventive genius to whom we are indebted for the phonograph. Mr. Edison has done this so effectively that he may be said to have given life to the zoetrope by the perfection in which the ocular illusion is produced together with the audible manifestations that keep time with it. The amount of thought and ingenuity expended on this new contrivance, which Edison has called the kinetoscope, will scarcely be appreciated by anyone who has not given some consideration to the many practical difficulties that have been overcome. No wonder that the announcement made at the beginning of 1892 should have been received with incredulity, for it was to the effect that Edison had contrived some happy combination of photography and electricity by which a man (presumably one who could afford to pay for luxuries) might sit in his own room and see the moving forms of the actors in an opera projected on a screen before his eyes, while at the same time he would hear their voices singing. Every movement, every change of expression, every glance of the eye, and, in fact, all that was visible to the spectator in front of the stage would appear on the screen, while not a note of vocalist, or chord of orchestra, would fail to reach the ear. And all this was to be evoked at will, and repeated as often as desired, not, therefore, of course, as a presentation of what was taking place at the time, but as a reproduction of some previous performance. This wonderful result has virtually been attained by the application of delicate and ingenious machinery designed to make the phonograph and the camera work synchronously. The first part of the problem was the production of a succession of so-called instantaneous photographs at an extremely rapid rate. In the actual apparatus forty-six photographs are taken every second, a feat which would beforehand be thought impracticable. This is accomplished by making use of a band of sensitive celluloid film, which alone admits of being moved and stopped with the desired rapidity. The movement is imparted by an electric motor, and the arrangement is such that for each exposure the film is held stationary for 9
10ths of 1
46th of a second, during which the lens is uncovered, then for the remaining ⅒th it is covered, while at the same time the film is jerked forward so as to expose a fresh surface to receive a new impression. Obviously the mass moved and stopped with this rapidity (which without the stoppages is at the rate of 26 miles an hour) must be small, and it is found that photographs about 1 in. in diameter cannot be much exceeded in view of this condition. The lens has to be entirely stopped or screened during the tenth of the short interval (1
460th of a second) in which the onward movement of the film is taking place, and it has to be practically open during the remaining 9
10ths of the interval (9
460ths of one second) in which the film is held stationary in order to receive the photographic image. These alternations of movement and stoppage must take place with the utmost regularity, and Edison has used a beautifully regulated electro-motor as the active power, which also simultaneously moves a phonograph so that sights and sounds shall proceed in step, for it is thus they have to be reproduced. This is done by developing the band of film, and from it printing photographic positives on a similar band, whose images are successively projected on a screen by means of a lantern with a step by step movement, exactly the same as that by which the original photographs are taken, while the phonographic cylinder is so timed as to give off to a loud-speaking instrument the sounds that accompanied the photographs. A description of the ingenious mechanism by which all this is accomplished is not suitable for these pages, for it is the result, rather than the details of apparatuses, that interest the general reader. In a simpler form of kinetoscope the positive images on the band of film are viewed directly by single observers, each looking through magnifying glasses; in this a disc with 46 slits revolves, and in its passage, as each slit momentarily permits a view of the image, an electric flash simultaneously lights it up. The same principle is, of course, used in the screen projections. From the very great number of impressions made on the eye in one second, there is none of that jerky movement that is observable in the older appliances. Mr. Edison has found it necessary to provide a special stage, or rather small theatre, in which the actors of the little dramas may be photographed with every advantage in the way of lighting, &c. Fig. [251a] shows this kinetographic theatre with the electric camera in action. The subjects reproducible in the kinetoscope include the most rapid movements, such as quick dances, blacksmiths hammering on an anvil, &c., or incidents of ordinary life involving much gesture and change of facial expression, and nothing can be more amusing than to see all these shown to the life by the images on the screen, or by the pictures viewed through the lens, especially if at the same time the phonograph is made to emit the corresponding sounds.
