Camera Lucida, an instrument invented by Dr. Wollaston, and constructed on the principle that when a beam of light in passing through a glass prism strikes an interior surface at an incident angle of more than 48° 30' it is totally reflected. Let A B C D, fig. 1, represent a transverse section of a prism having the sides A D and D C at right angles, the angles A and C each 67° 30', and the angle at B 135°. If now a ray of light enter the prism from b perpendicular to the face D C, and of course parallel with A D, it will pass without refraction to the point a, where it will be totally reflected in consequence of the angle of incidence bad being greater than 41° 48'. In this case the angle of incidence will be 67° 30', so that the sum of the angles of incidence and of reflection, b a c, will be 135°, and equal to the angle ABC. The reflected ray a c will therefore strike the face A B at c with an angle of incidence of 67° 30', and suffer reflection in the line c d perpendicular to the face A D (360° - 4 x 67° 30'=90°). If the eye could be placed anywhere on the line a e, it would perceive an inverted image at a of an object at b; but when the eye is placed at d, an erect image is seen in consequence of the two reflections.

But as the instrument is intended as an aid in drawing, it is necessary to have an image of the object projected on a sheet of paper. This is effected by placing the eye so near the edge of the prism that the image and the paper may be seen at the same time, as in fig. 2. In consequence, however, of the image appearing further from the eye than the paper, the use of the instrument is attended with considerable practical difficulty, which is only partially overcome by placing a convex lens between the eye and the prism. Few persons ever succeed in obtaining satisfactory results, while some acquire considerable facility of manipulation. - Another form, devised by Amici, is more manageable than Wollaston's, in consequence of not confining the eye to one particular place. A right-angled triangular prism, in place of a quadrilateral one, is employed, which produces two refractions and one reflection. This is represented in fig. 3. The prism is so placed that one of its sides bounding the right angle is perpendicular to a plate of glass which is used as a reflector.

Those rays of light proceeding from the object at 5, which are caused by refraction to strike the base of the prism, are reflected by its internal surface to the opposite side of the prism whence they issue, refracted from the perpendicular, and are again reflected by a glass plate to the eye, forming an image which appears, as if at c, where the point of a pencil may trace its outline on paper. Fig. l. Fig. 2. Fig. 3.