PHENAKISTOSCOPE Synonym, stroboscope. An instrument in vented simultaneously, by, Plateau, of Ghent, and Stampfer, of Vienna, in the year 1832. It has been made in various forms, an example of which is illustrated. Two discs are mounted on a common spindle ; they rotate in the same direction and at an equal speed. One disc is perforated with slots, through which an observer may view a series of pictures depicted on the inner surface of the companion disc. The series of pictures consists of successive phases of any object in motion ; as for example, d girl skipping. The number of phases corresponds to the number of slots in the viewing disc. On making the discs rotate rapidly, the observer gains n momentary view of each picture in the series as each slot arrives opposite its respective image. In virtue of the law of persistence of vision, therefore, 4 blending of all the pictures in the series takes place on the retina of the eye, giving the impression of a single figure invested with life or motion. In a modified form of the instrument, the disc bearing the pictures, as shown in the figure, is dispensed with, by depicting the pictures on one side of the slotted disc and viewing them through the slots by reflection in a mirror ; this, indeed, was the earliest form of the apparatus. Thenakisto scope is the name given to the instrument by Plateau, while Stampfer calls it by the name of stroboscope. As an early instance of confusion of terms it may be mentioned that Snell, writing in 1835, calls the stroboscope by the name of " Phantascope" or "Xaleidorama." Miiller, in 1846, applied this instrument for the demonstra tion of wave-motion, and Poppe, Savart, and others employed it for the synthesis of other natural motions. The first attempts at projecting
animated effects upon the lantern screen were founded on the type of machine here illustrated, the disc bearing the series of pictures being on transparent material, and light thrown first through it, then through slots, and finally on to the screen by means of an objective. This was done by Uchatius between 1851 and 1853, but Plateau himself had practically attacked the same problem in 1849 in a modification of his anorthoscope, in which apparatus he produced four non-distorted images from a distorted original by the introduction of compensating lenses. Plateau placed sixteen images in pro gressive series round the margin of a glass disc, and in front of this, in a reversed direction, revolved, at a four times greater speed, an opaque disc with four slots. The front of the apparatus could be observed by many people at once, and to prevent confusion the parts of the disc showing the non-erect images were screened off. It will be seen that as a slot passed the aperture in the screen one image would be viewed and the light then cut off while the transparent disc turned one-sixteenth of its diameter and the opaque one one - quarter. The next image would then be revealed, by its coincidence with the slot, in the same position as that in which the previous image was observed. (See also " Zoetrope.")