Extra glass prism, for tripod stand mounted camera obscura
Extra prism for camera obscura in tripod stand
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.