[Illustration: Correspondance Mathématique, 1829–1833 _PLATEAU’S first real motion picture device, shown above, see page 89. Below, the Phénakisticope with which a single person could see pictures in motion._] The first machine creating the illusion of motion from a series of drawings was described by Plateau in a letter to Quetelet dated Liége, December 5, 1829, with the scientific title, “Different Optical Experiments.” (_Relative à différentes expériences d’optique._) A similar instrument was already referred to by Plateau in his paper written in the preceding year. Although the device made by Plateau in 1828 and described in the 1829 article followed by several years the introduction of the Thaumatrope, it rates as the first motion picture machine because the Thaumatrope was really only a scientific toy, just as Paris called it. Plateau illustrated his letter describing his instrument in writing to Quetelet in answer to an inquiry. The drawing (opposite page) of Plateau shows that, though a scientist, he never forgot his early training and was something of an artist. The principles of his machine could be illustrated by drawings of lines and other geometrical figures, but Plateau chose a woman’s head. In the following words Plateau described his instrument: Two small copper pulleys, (a) and (b), drive by means of an endless cord a large wooden wheel, (c), which has a double groove; the diameters of the small pulleys are such that the two cords are equally taut and the system is placed in movement by means of the handle, (d), the speed of one pulley being an exact multiple of the other; the axes terminate in the form of a vise and are divised in such a way that you can attach to them by little screws the drawings or cartoons with which you wish to experiment. The pulleys are held by iron supports, (f) and (g), which slide in two grooves practically parallel with the stand or base (hk), and are held in position by means of thumb screws. Lines or drawings to be studied are mounted on the two pulleys. The machine is of such a nature, Plateau pointed out, that drawings can be easily changed, the relative speeds of the two wheels (one serving as a shutter when drawings are used) can be regulated, alignment can be readjusted and by crossing the cords the disks can be made to rotate in opposite directions. Plateau continued by explaining that when the speed on one disk is not an exact multiple of the other they do not keep the same relative positions after rotation. A different image is produced at each revolution and the eye, instead of seeing one fixed line (or image), sees only a rapid succession of different lines (or images); however if the swifter is little more than a multiple of the other, the difference is very little in a manner which the eye cannot distinguish one from another. In this case the spectacle will appear to change little by little....