Zograscope

A zograscope is an optical device for enhancing the sense of depth perception from a flat picture. It consists of a large magnifying lens through which the picture is viewed. Some models have the lens mounted on a stand in front of an angled mirror. This allows someone to sit at a table and to look through the lens at the picture flat on the table. Pictures viewed in this way need to be left-right reversed; this is obvious in the case of writing. A print made for this purpose is called a vue d'optique or perspective view.

According to Michael Quinion, the origin of the term is lost, but it is also known as a diagonal mirror, as an optical pillar machine, or as an optical diagonal machine.

Zograscopes were popular during the later half of the 18th century as parlour entertainments. Most existing ones from that time are fine furniture, with turned stands, mouldings, brass fittings, and fine finishes.

In 1677 German writer Johann Christoph Kohlhans described the effect of a convex lens in a camera obscura as if the subject appeared "naked before the eye in width, breadth, familiarity and distance".

in 1692 William Molyneux wrote in his Dioptrica Nova how "Pieces of Perspective appear Natural and strong through Convex Glasses duly apply'd".

The zograscope was developed around 1745 in England and soon many perspective views were printed for it, mostly of views with urban architecture. The oldest known reference to the device is found in an advertisement in an English newspaper from April 1746.

One way a zograscope enhances depth perception is by minimizing other depth cues that specify the flatness and pictorial nature of the picture. The image is magnified, perhaps giving it a visual angle similar to the real scene the picture is depicting. The edges of the picture are blocked by the frame of the lens. The light coming from the lens to the eye is collimated, preventing accommodation.

A second way a zograscope enhances depth perception is by creating stereopsis. Because each eye views the image from a different position, the visual directions of contours in the picture might differ for each eye, creating horizontal disparity. As well, coloured parts of the image will be refracted differently for each eye, creating a version of chromostereopsis.

...
Wikipedia