Camera lucida

A camera lucida is an optical device used as a drawing aid by artists and microscopists. By looking through the prism in its standard, a user sees an optical superimposition of the subject positioned in front of the device over the surface below. This allows the artist to duplicate key points of the scene on the drawing surface, thus aiding in the accurate rendering of perspective. A camera lucida is somewhat similar in mechanism to a periscope. However it completes two total internal reflections, generally within a single glass parallellepiped or rhomboid that works as a double prism. This causes a shifted (parallax-like) image of the pencil tip that overlaps with the microscopic field of vision. Then the microscopist traces the contours of the objects seen in the microscopic field on the piece of paper using the pencil.