CAMERA LU'CIDA and CAMERA OB SCU'RA (light and dark chambers) are names given to two methods, very like in princiPle,1 of throwing images of external objeets upon a plane or curved surface, for the purpose of drawing or amusement.
The Camera Obscura now in use has, oc casionally, the form of a box. In ft-tint is a sliding tube, carrying a convex lens, through which the light from distant objects passes to a mirror at the opposite extremity of the box : the mirror is inclined to the horizon at an angle of 45 degrees, and from thence the light is reflected upwardS to a glass plate in a hori zontal position. The rays in the pencils con verge at the upper surface of this plate, which, on that surface only, is ground rpugh, and thus the images of distant objects are visible upon it, a shade over the plate preventing the direct light from interfering with theni A camera Oscura for exhibition is geperAy made in a room with a panic* roof and an aperture at the Abeye this aperture is a revolving plane inirror ippling4 at and reflecting pencils 4!:caliWards. A eclY9X lend causes these pencils to converge linen a sur face of plaster of Faris, properly curved. The mirror revolves about a vertical axis, thus allowing all the compass points of a landscape to he successively thrown pp the suifface.
Pertable camera RI)Senraa ace often made in a similar manner, the mirror and lens being in a sliding ease at the top of a pyra midal box, and the image being received on paper laid the bottom. Apertures en one
side of the box allow the spectator to see the image and introduce a band for the of drawing op the paper.
A camera Oscura is an indispensable aid in most forms of photographic operation.
Thp amtertt Le!cidif was by Dr. Wollaston. It consists of a metal stand sup porting a glass prism, of which one angle is Iv. The action pf the instrument is such, when re3T of light fall horizontally on one side of the nrism; they suffer reflection within the glass, and are thrmvn upwards to the eye through ari (men* in a plate which excludes all except the end of the-prism, and a part of a sheet of paper or ether flat snrface placed beneath the prism. mpncp the image of an object is thrown towards the visible part of the paper, and, the eye viewing both the image and the sheet of paper (with different parts of the pupil, however, which creates a difficulty in using this instrument), the ob• server is enabled to trace the object upon the Paper The image of the distant object most be made to coincide with that of the paper ; and, for this purpose, since, expept when the object is very remote, the rays in the pencil are in a divergent state, a convex lens is interposed between the prism the paper.