CAMERA OBSCURA (Lat. ((dark chatn an optical instrument employed for ex hibiting the images of external objects in their forms and colors, so that they may be traced and a picture formed. From certain scattered observations in the writings of Friar Roger Bacon, in the 13th century, it would appear that he was acquainted with the principle upon which the camera obscura is constructed, but the first complete description of this instrument is found in the c Magia Naturalis' of Giambat tista della Porta, published in 1569, and Porta is commonly credited with its invention.
In its simplest form the camera obscura con sists of a darkened chamber, into which no light is permitted to enter excepting by a small hole in the window-shutter. A picture of the objects opposite the hole will then be seen on the wall, or a white screen placed so as to receive the light coming through the opening. The images thus obtained become sharper as the size of the hole is diminished; but this diminution involves loss of light, so that it is impossible by this method to obtain an image at once bright and sharp. This difficulty can be overcome by placing a lens in the opening in the shutter. If the objects in the external landscape are all at distances many times greater than the focal length of the lens, their images will all be formed at sensibly the same distance from the lens and-may be received upon a screen placed at this distance. The images are inverted, and
are of the same size whether the lens is in position or not, so long as the screen remains fixed; but they are far sharper and more dis tinct when the lens is used. As an attraction at seaside resorts and other places of amuse ment, the camera obscura consists of a small building or of a tent surrounded by opaque cur tains and having at its top a revolving lantern, containing a lens with its axis horizontal, and a mirror placed behind it at a slope of 45°, to reflect the transmitted light downward of a sheet of white paper lying on the top of a central table. Images of external objects are thus depicted on the paper, and their outlines can be traced with a pencil if desired. It is still better to combine lens and mirror in one by the arrangement represented in section in the figure. Rays from external objects are first refracted at a convex surface, then totally re flected at the back of the lens (or prism), which is plane, and finally emerge.through the bottom, which is concave, but has a larger radius of curvature than the first surface, The two re fractions produce the effect of a converging meniscus. The camera obscura. which was for merly chiefly empliiycd for purposes of amuse ment, has now become well known from its adaptation to photography. Sec PHOTOGRAPHY.