CAMERA LUCIDA (Lat. "light cham ber"), an optical instrument employed to facili tate the sketching of objects from nature. It acts by total reflection, and may have vari ous forms, of which that proposed by Wol laston, and illustrated in the accompanying figures, is one of the commonest. The es sential part is a totally reflecting prism with four angles, one of which is 90°, the bite one 135 and the other two each 67° One of the two faces which contain the right angle is turned toward the object to be sketched. Rays falling in a straight line on its face, as x r, are total) ." reflected ;Imam face r d to the next fate d a, whence they are again totally reflected to the fourth face,. from which they emerge in a straight line. The eye (pp) placed so as to re ceive the emergent rays will see an image of the object in a dace" tion of right angles to that in which the object lies. In practice the eye is held over the corner a of the prism in such a position that one-half of the pupil receives these reflected rays, while the other half receives light in a parallel direction outside the prism. The observer thus sees the reflected image projected on a real background, which consists of a sheet of paper for sketch ing. He is thus enabled to pass a pencil over the outlines of the image — pencil, image and paper being simultaneously visible. It is very desirable that the image should lie in the plane of the paper, not only because the pencil-point and the image will then be seen with the same focusing of the eye, but also because parallax is thus obviated, so that when the observer shifts his eyes the pencil-point is not displaced on the image The introduction of an ordinary gstopp as used in photography remedies this defect, although greatly reducing the volume of light passing. As the paper, for convenience of drawing, must be at a distance of about a foot, a concave lens, with a focal length of some thing less than a foot, is placed close in front of the prism in drawing distant objects. By rais
ing or lowering the prism in its stand (Fro. 2), the image of the object to be sketched may be made to coincide with the plane of the paper. The prism is mounted in such a way that it can be rotated about either a horizontal or a vertical axis; and its top is usually covered with a movable plate of blackened metal, having a semi-circular notch at one edge for the ob server to look through.
Another form of the camera lucida, that of Amici, an Italian optician, is sometimes pre ferred to that of Wollaston, inasmuch as it al lows the observer to change the position of his eye considerably without ceasing to see the image of the object he is tracing. The prism in this case is triangular in shape, and one of the angles is a right angle. In using it, the right angle is turned upward, so that one of the per pendicular fates is turned toward the object in an oblique direction, while the edge of the other perpendicular face meets a transparent glass plate at right angles. The rays from the object falling upon the face of the prism which is turned toward it are, after being more or less refracted, thrown upon the base of the prism, from which they are totally reflected in the direction of the other perpendicular face. In emerging from the prism of this face they are again refracted and thrown upon the trans parent glass plate. By this, again, the rays are partially reflected, being thrown upward in the direction of the eye of the observer, who, looking through the plate, sees an image of the object, on a sheet of paper beneath, the outlines of which can be traced by a pencil as before. Perhaps the most important use of the camera lucida is its adaptation and attachment to the Microscope; but here also direct photography has almost wholly superseded handwork.