Camera Lucida

a prism of glass held in a frame. Its faces are cut at such an angle that an object lying at one side, when viewed through the prism, seems to lie on a sheet of paper directly underneath. The instrument is much used in drawing natural objects. A pencil may be made to follow the apparent outline on the paper with ease. The pencil of light enters the prism horizontally, is totally refracted twice, and enters the eye, passing directly upward at right angles to the original direction.