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2G LIVING PICTURES. appears compressed in the direction of its length. Plateau in 1849 had recognised this difficulty, and therefore prepared his diagrams in a form purposely distorted in an opposed sense to the distortion caused by the revolving disc, one distortion thus neutralising the other. This defect led Clerk- Maxwell, in 1869, to propose the substitution of concave lenses for the slots, their focal length being equal to the diameter of the cylinder. The virtual image of the design opposite the lens was thus formed exactly midway between lens and picture, and this spot necessarily coincided v/ith the axis of rotation. That being the case, the successive images were perceived in one and the same spot, and remained stationary during the whole time they were individually exposed to view, the movement of the lens being neutralised by the movement of the real object on the other side of the cylinder. It will be seen that the distortion common to all ordinary types of slotted machines was thus done away with, and at the same time the images appeared more brilliant—a wide lens being substituted for a narrow slot. Maxwell used this device for combining series of diagrams of many physical phenomena (such as smoke-rings, etc.), in order to show the resultant movement, but the apparatus does not seem to have come into general use. In the year 1877, however, Rey- naud patented a contrivance which .attained almost instant popularity under the name of the Praxino- scope (Fig. 22). In this instru- ment the pictures are not directly viewed, but are seen in a mirror, the picture under observation thus being the one nearest the observer instead of that on the opposite side of the cylinder.. FlO. 22.