This is probably the most reliable optical device employed for copying. The principle of its construction will be understood in the diagram marked 2 in the engraving. The glass is simply a four-sided prism, having one right angle, one of 135°, and two of 67-1/2°. When disposed as represented, the rays from the object pass into it without any appreciable refraction, and are totally reflected from the lower inclined side, and again from the upper inclined side, emerging near the summit in a direction almost perpendicular to the top face, so that the eye sees on the paper placed beneath an image of the object. If the image be traced by the pencil, a very correct outline, not reversed, is obtained. The use of the device requires practice. The nearer the object copied is brought to the prism, the larger is its image, and vice versa.

Fig. 1 -The Camera Lucida

A simple method of constructing the camera lucida is shown in Fig. 2, and is the invention of Mr. H. E. Mead, artist of the Scientific American. The prism can be obtained at a small cost from any optician, and the rest of the apparatus any one can cut out of black walnut with a knife, and perhaps a gimlet. The thumbscrews used are of brass, of the kind employed for shutter-fastenings, and can be procured of any hardware dealer for a few cents each. B is the prism, and A is a section of one of the joints, showing how the apparatus may be easily adjusted. A movable rod, secured by a thumbscrew, regulates the height of the prism, and the single clamp shown secures it to the table. The cost of the whole is about seven dollars.