CAMERA OBSCURA (Fr., Chambre obscure ; Ger., Die Dunhelltammer) Literally, " dark chamber " ; an optical instrument invented by Baptista Porta in 1569, although there is evidence of an even earlier knowledge of its principle and properties. This simple instrument depends in principle on the fact that if a tiny hole is made in the shutter of a room from which light is otherwise excluded, a small reversed image of the view outside will, under favourable circumstances, be thrown on the opposite wall. This experiment appears to have been known to philosophers from time immemorial, but only comparatively recently was discovered the improvement effected by using a convex lens in place of the hole. Baptista Porta's box-form of camera obscura appears to have been used as an entertaining toy, or as a ready means of tracing landscapes and views, for nearly three centuries prior to the discovery of photography. A quaint form of camera obscura, designed by A. M. Guyot, for outdoor work in tracing land scapes, is shown in section at A. It resembles an ordinary table, the camera being situated between the legs and the top being formed by a sheet of plain glass as, on which is laid a piece of tracing paper. The image formed by the convex lens n is thrown upwards on the screen M by being reflected from an inclined mirror A modification of this device is shown at B, the projected image being viewed under cover of a dark chamber, at the top of which the optical system is arranged. A double convex lens is placed in a sliding mount at lc, and over it is a mirror r„ set at an angle of 45° relative to the horizon. As the lens is uncorrected for spherical aberration, the image would suffer in definition at the margin if received upon a perfectly plane surface. Therefore the surface as is made con cave, and part of a sphere whose radius is the focal distance of the convex lens K.
The best form of camera obscura is that in which internal instead of specular reflection is employed, to prevent the loss of light attendant on the latter. The optical system then con sists of a rectangular prism C, having one of its faces convex and another concave, such a com bination doing away with the necessity of a mirror to change the direction of the rays from The box-form camera obscura is shown at F, and it will be seen that the principle here employed is practically the same as that of Guyot's table-form apparatus, with the addition of a shade Q. This device is sometimes used by a horizontal to a vertical course. The rays from a distant object or landscape will be made to converge after impinging on the convex surface, and being reflected in the interior of the prism, will pass into the dark chamber to the surface upon which the picture is to be received. The
picture thus obtained will be extremely vivid. With an optical system of this character, the surface on which the picture is formed may be plane and not concave. As these meniscus prisms are difficult to procure, they may be replaced by a triangular prism N (see illus tration D), having a plano-convex lens o and a plano-concave lens P, both of proper focal length, cemented by Canada balsam on two of its faces. Spherical aberration is sometimes guarded against by using a plano-concave lens E in place artists in sketching or, rather, tracing outline pictures of landscapes. x is the lens, t, the mirror, and m the sheet of tracing paper, or ground glass with the matt surface uppermost The modern photographic camera obscura which will be readily recognised as the reflex or reflector camera—is arranged in the same way as F, but the lens used at x is of the compound type, in which spherical aberration, achromatism, and all the other defects of a simple lens are corrected ; so that when the mirror L is mechanic ally moved out of the path of the rays a perfect negative image is received upon the sensitive plate, suitably placed at the back of the camera.
A sterescopic camera obscura devised by Theodore Brown is a half-plate instrument fitted with a mirror for reflecting the rays on to a horizontal screen where the stereoscopic images are seen. Unlike an ordinary stereoscopic camera, in which a pair of lenses side by side are used, only one lens is used, but it is supplemented with a double reflecting device (the " Stereo photoduplicon," which see), to be used on the hood of the single lens. The effect is very of the more complex combination, in which case the lens is placed at the top of the dark chamber with its concave surface uppermost. With this latter arrangement a plane surface suffices to receive the picture, but the mirror 1, in illus tration B will still be needed to turn the rays from the horizontal to the vertical direction.
charming, especially when the face of the observer is properly enclosed within the hood or shade placed above the screen on which the dis similar images are projected. By turning the camera on its axis during inspection of the images a panoramic, as well as a stereoscopic, natural colour effect is produced.