The Optical Magic Lantern Journal (July 1895)

The Optical Magic Lantern Journal and Photographic Enlarger. 113 example of persistence of vision is seen when we take a piece of string, light one end, and, with the glowing spark obtained, twirl it round, when we shall have, if the string has been twirled fast enough, a circular ring of light. This is because the points of light are so rapidly brought to our vision that the previous ones have not had time to die away. Plateau made a number of experiments and came to the conclusion that the average duration of an impression upon the retina of the eye was half asecond, but although the duration of the impression upon the retina of the eye is half a second, still a flash of light occupying a very small fraction of that time is sufficient to: give that impression. The action of a running horse as is usually painted by artists is erroneous when looked at from a scientific point of view—that is, when photographed in a fraction of a second. The same results would be seen if instead of photographing the running horse in a small fraction of a second we were to suddenly illuminate it with a flash of light ; we should then see the horse as it were stationary, as when a flash of lightning illuminates the surrounding objects at night. The stroboscope, a simple apparatus which has been devised and adapted to the lantern, enables us to make objects that are really in motion appear to be stationary, or moving at a much slower rate. The stroboscope described later is so simple a piece of apparatus that any one possessing a lantern should make one, as by studying its principle and the application of it for physical research much knowledge can be thus gained. The apparatus consists of a circular disc of card or metal about six inches in diameter with six equidistant holes about half an inch from the edge. This disc of card or metal is fixed in the focus of the condensing lens —removing the objective—as shown in Fig. I., and is made to revolve as regular as possible, the speed varying with the experiment you are applying it to. To obtain a uniform speed it is necessary that the disc should be heavy. and composed of metal, but when the motive power driving the disc has its own fly-wheel attached the heavy metal disc can be dispensed with and card substituted. For rough experiments the disc could be turned by hand as by the apparatus described, but in some of the more interesting experiments a motor of some description giving a regular speed of motion must be resorted to. A motor, such as a small electric motor, which can be obtained very cheaply, would serve the purpose, as the speed can be easily varied. Figs. II. and III. are suitable for use with ‘the hand. Fig. II. isan edge view and Fig. III. a Fig. II. Fig. III. front view. Two uprights aa are strongly fixed in a base x, and at the top of the uprights aa an axis C passes through, which axis has mounted upon it a circular disc p with the holes around the circumference equidistant apart. It is necessary to have the holes equidistant apart, otherwise the flashes of light will not occur at regular intervals—as is required. Upon the axis cis a small pulley © having a cord F passing over it, and around another pulley 1 somewhat larger mounted on an axis G, below the axis c, and low enough for the disc p to clear itself and have free movement. Upon the axis Gis arrangeda handle u for giving motion to the wheel. The whole should be made to run smoothly and freely without jerking. Such an experiment as the following, which was Shown during a course of lectures at Jermyn Street Museum by Professor ©. V. Boys, gives an example of the kind that can be shown by the stroboscope. The objects experimented upon were jets of water. The jet of water from a small nozzle was brought under the influence