Camera Obscura in tripod stand with extra prism

This apparatus, which dates from the first half of the 19th century, is a modification of the ordinary form of mirror instrument. It was extensively used by artists in sketching from nature, before the introduction of photography.

The camera consists of a pyramidal box supported on a tripod which also carries an optical system for the production of the image.

A brass tube open on one side moves in another tune and contains a glass prism, one face of which (AC) is convex, and the other (CB) concave. Light entering the opening in the brass tube, falls on the convex surface, enters the prism, and being totally reflected at the plane face AB, passes into the box through the concave surface, and falls on a white surface at the base of the chamber.

The picture thus obtained is free from spherical aberration, a advantage not obtained with the mirror and convex lens of the ordinary instrument.

This totally reflecting prism is designed for use with the prism form of camera obscura, an example of which is exhibited in an adjacent position.

It consists of a rectangular prism, the two small faces of which are unequally convex, Light incident on one of the curved surfaces enters the prism and is reflected at the plane surface. It passes out of the prism through the other curved surface and is brought to a focus, the two curved portions of the prism functioning as a lens.

Generally, one of the prism surfaces is concave, in which case the image is produced at the further distance from the prism than when both surfaces are convex.

As the prisms are difficult to make, the same result sometimes attained by cementing with Canada balsam a plano-convex lens on one side of a rectangular prism and a plano-concave one on the other.