Camera Obscura

(ob-skū'ruh.) [L., dark chamber.] (Opt.) It is, as its name implies, a closed space impervious to light. There is, however, a small aperture by which luminous rays enter. The ray, proceeding from external objects, and entering by this aperture, forms on the opposite side an image of the object in its natural colors, but of reduced dimensions, and in an inverted position. Porta, the inventor of this instrument, a found double that by convex fixing lens in the aperture, and placing awh ite B screen in the focus, the image was much brighter, and more definite. Fig. 123 shows a kind of C. which is occasionally erected in summer houses. In a bra case, A, there is a triangular prism which acts both as condensing lens and Fig. 123.- cAMERA OBSCURA as mirror. One of its faces is plane, but the others have | such curvatures that the combined refractions, on entering and emerging from the prism, produce the effect of a meniscus lens. Hence, rays from an exterior object, after pa ing into the prism, and undergoing total reflection, form an image on a piece of paper placed on the table or focus, B. The C. is now almost entirely confined to photographic purposes, and the shapes and forms are varied according to the requirements of almost each operator. [s. 133]