A camera lucida is an optical device used as a drawing aid by artists and microscopists.
In this video Robert Murray Smith shows how you can make your own for next to nothing by recycling an old LCD projector.
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.