CAMERA OESCURA (literally, a dark chamber), an instrument invented by Baptista Porta in the 16th century. It is known in its simplest form as a familiar toy, con sisting of a rectangular box, furnished at one end with a lens whose focal length is equal to the length and depth of the box; at the opposite end of which a plane reflec tor is placed at an angle of 45°, which throws the image of any objects to which the lens may be directed on a piece of ground-glass on the top of the box in a non-inverted position, so that they may be viewed or sketched from as in nature.
The C. 0. being now an indispensable article in the practice of photography, has received a number of recent improvements, which make it rank as a scientific instru ment. The principle, however, involved in the simplest and most refined forms is the same, and may be illustrated and made intelligible by the following experiment: Let a small hole be bored in a window-shutter, and the room be darkened. If, now, the beam of light entering the room by this hole be intercepted by a sheet of white paper, held at a small distance from the hole, an inverted image of objects without will be seen upon the paper. By placing a small convex lens over the hole, this image is ren dered much more distinct, or sharp, in photographic language. it will be found that, at a certain distance from the hole, the image attains a maximum degree of sharpness; and that if the paper be removed from this point to any position either nearer to the hole or further from it, the image becomes indistinct and confused. At the point of greatest distinctness, the image is said to be focused. Such being the principle of the camera, it is evident that in practice the instrument may assume many forms, provided always that it consists of a darkened box or chamber having a hole at one end for the insertion of a lens, or combination of lenses, and at the other a screen, generally made of ground-glass, on which to receive the image.
The body of the instrument may be made of any opaque substance; the tube or tubes are generally formed of brass, and contain one or more lenses; there is the obscured or ground-glass, upon which the image is thrown for the purpose of adjusting the focus; mid the rack behind, by means of which, and the double sides of the camera, the body of the instrument may be lengthened or shortened till the image on the ground screen is accurately focused. This rack is most frequently placed upon the tubes car rying the lenses. The interior of the whole apparatus is blackened, to prevent reflec tion of the rays falling on their sides, and to impart greater distinctness to the picture.
The camera-slide is a thin, dark box, and is used for conveying a sensitive plate from the operating-room to the camera, and back again after exposure. It consists of a rec tangular frame, made to fit exactly into the back of the camera when the focusing screen is removed. At the back is a hinged door, by means of which the plate is intro duced into the slide: and in front is a shutter, which is pulled up when the plate is to be exposed, and shut down after the time requisite for the action of the light upon the plate has expired. It must be constructed so that, when substituted for the focusing
screen, the surface of the prepared plate, which is intended to receive the image, shall correspond exactly in distance from the lens with the ground-surface of the focusing screen. The plate rests upon projections of silver wire in the corners of the slide; and the same slide may be used for plates of different sizes, by introducing into it thin frames of suitable dimensions also furnished with silver-wire corners.
Photographic cameras are generally required for one of three purposes—viz., portraits, landscapes, or copying; and for each of these it is necessary to make suitable modifica tions in the construction of the instrument. A camera has, however, been recently contrived which combines within itself the conditions necessary for all contingencies. It is called Martin's universal portrait, landscape, and copying camera, and consists, primarily-, of a base-board, 30 in. long and 11 in. wide, divided into three pieces, and hinged together by means of broad brass hinges, so as to diffuse the bearing as much as possible, and bolted together when in use by sliding panels of mahogany, extending across the entire width of the base-board. This base-board being grooved on its outer edges, allows the sliding portions of the camera to be moved from one end to the other, so as to alter the relation between object, lens, and image ad infinitum.
What may be regarded as the body of the camera, is of the same construction as an ordinary expanding camera, except that it is furnished with additional apertures for camera slides, and the front and the back are united by means of an accordion or bel lows body of suitable length to extend from one end of the base-board to the other.
Liebhard, which name he changed into C. ,because his forefathers had been Ri'immerer (chamberlains) at the court of the bishop of Bam berg—was born at Bamberg. April 12, 1500, and died at Leipsic, after a life devoted to literature, April 17, 1574. He was by nature earnest and taciturn; but the extent of his knowledge, his sobriety of opinion, strength of character, and, when he pleased, over powering eloquence, won for him the esteem of all his contemporaries. His works, of which several still remain valuable, include an excellent biography of Melanehthon, and a collection of letters by this reformer; also annotations on Cicero's Qumstiones Tuseu law (1525); Elements of Rhetoric; Commentarii Lingua G•ace et Latince (1551) ; and Bin:v Familiares (1583-95), giving interesting notices of his times.—His son, JOACHIM C. (b. 1534, d. 1598), was one of the most learned physicians and botanists of his age.