You searched for: “anorthoscope
1. An optical toy for producing amusing figures or pictures by means of two revolving disks, on one of which distorted figures are painted.
2. An apparatus for producing and studying anorthoscopic perception which is the real perception of an image viewed under very uncommon conditions.

It consists of a pair of circular discs mounted one behind the other and a figure is drawn on the rear disc and viewed through a slit in the front disc rotating at a different speed while the shape of the image is clearly seen.