The Probert Encyclopaedia of Science & Technology

CAMERA LUCIDA

A camera Lucida is an optical instrument employed to facilitate the sketching of objects from nature by producing a reflected picture of them upon paper. Wollaston's apparatus is one of the commonest. The essential part is a totally-reflecting prism with four angles, one of which is 90 degrees, the opposite one 135 degrees, and the other two each 67 degrees 30 minutes. One of the two faces which contain the right angle is turned towards the object to be sketched. Rays falling in a straight line on this face, are totally reflected from the face to the next face whence they are again totally reflected to the fourth face, from which they emerge in a straight line. The operator's eye placed so as to receive the emergent rays will see an image of the object and by placing the sketching paper below the image may be traced with a pencil. As the paper, for convenience of drawing, must be at a distance of about a foot, a concave lens, with a focal length of something less than a foot, is placed close in front of the prism in drawing distant objects. By raising or lowering the prism in its stand, 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 either about a horizontal or a vertical axis; and its top is usually covered with a movable plate of blackened metal, having a semicircular notch at one edge, for the observer to look through. This form of camera has undergone various modifications. It is very convenient on account of its portability.
Research Camera Lucida