Camera lucida

A camera lucida is an optical device used as a drawing aid by artists.

The camera lucida performs an optical superimposition of the subject being viewed upon the surface upon which the artist is drawing. The artist sees both scene and drawing surface simultaneously, as in a photographic double exposure. This allows the artist to duplicate key points of the scene on the drawing surface, thus aiding in the accurate rendering of perspective.

The camera lucida was patented in 1807 by William Hyde Wollaston. The basic optics were described 200 years earlier by Johannes Kepler in his Dioptrice (1611), but there is no evidence he or his contemporaries constructed a working camera lucida. By the 19th century, Kepler’s description had fallen into oblivion, so Wollaston’s claim was never challenged. The term "camera lucida" (Latin "light room" as opposed to camera obscura "dark room") is Wollaston's. (cf. Edmund Hoppe, Geschichte der Optik, Leipzig 1926)

While on honeymoon in Italy in 1833, the photographic pioneer William Fox Talbot used a camera lucida as a sketching aid. He later wrote that it was a disappointment with his resulting efforts which encouraged him to seek a means to "cause these natural images to imprint themselves durably".

In 2001, artist David Hockney's book Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters was met with controversy. His argument, known as the Hockney-Falco thesis, is that the notable transition in style for greater precision and visual realism that occurred around the decade of the 1420s is attributable to the artists’ discovery of the capability of optical projection devices, specifically an arrangement using a concave mirror to project real images. Their evidence is based largely on the characteristics of the paintings by great artists of later centuries, such as Ingres, Van Eyck, and Caravaggio.

...
Wikipedia