CAMERA LIICIDA, an optical instrument constructed of various forms, and for various purposes. Dr. Wollaston's C. L., intended to facilitate the perspective delinea tion of objects, consists of a small quadrilateral prism of glass, of which AR in the annexed figure is the perpendicular sec tion, held in a brass frame, which is at tached to an upright rod, having at its lower end a screw-clamp, to fix it to the ;1 edge of a table. The prism being at the height of about a foot from the table, has its upper face horizontal. Two of its faces, as in the figure, are at a right ... .... . . . .
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4...,,,..---. y'1 1 1 . angle at A; the contiguous faces make ........_ respectively with them :angles of so Q that the remaining obtuse angle at B contains 135°. Rays coming from an object PQ, and falling nearly perpendic ularly ou the first surface, enter the ,p q prism, and undergo total reflection at Camera Lucida. the contiguous surface (see Drprritics); they then fall at the same angle on the next surface, and are totally reflected again; finally, they emerge nearly perpendicularly to the remaining surface. An eye, as in the figure, then receives the emergent pencil through one part of the pupil, so that an image, pq, of the object is seen projected upon a sheet of paper upon the table. The rays from the drawing-pencil passing the edge of the prism, enter the other part of the pupil; and the pencil and image being seen together upon the paper, a sketch of the latter can be taken. There is, however,
a practical difficulty—the image and the drawing-pencil are at distances sensibly different from the eye, and so cannot be seen together distinctly at the same time. To obviate this, a plate of metal, with a small aperture as an eye-hole, is placed at the edge under the eye, so that the rays through the prism, and those from the drawing-pencil, which both pass through the eye-hole, form only very small pencils. By this, the difficulty is greatly diminished. It is still, however, difficult to use the instrument satisfactorily; and though many acquire great readiness in its use, others have never been able to attain i the same facility. The instrument is remarkable for its small bulk and portability. A good one will pack in a box 8 in. by 2, and i in. deep. Besides this form of the C. L., which is the most common, there are others. Its simplest form is merely a piece of smooth glass fixed at an angle of 45° to the horizon. An image from a horizontal object falling on this glass will be perfectly reflected, and that in the vertical, so that the eye looking vertically down will see the image, and, owing to the transparency of the glass, the artist will be able to trace it out upon paper below. In this case, how ever (see CATOPTIIICS), the image will be inverted.