CAMERA OBSCU'RA (Lat., dark chamber). A light-tight box, with a convex lens at one end and a screen at the other, on which an image is produced. This screen is generally a piece of ground glass, or translucent paper, so that the image may be viewed from behind. The rays of light coining from an object pass through the lens, and when they reach the screen form an image which can be received on a sensitive plate and preserved in the form of a negative. If the screen is placed at the appropriate focal distance, a sharp representation of an object can be obtained. The human eye is a simple form of camera obseura, the crystalline lens and retina occupying the relative positions of lens and screen. The lens is not essential for the forma tion of an image, as with a small opening a picture is also produced. Such an image, while free from distortion, is not apt to he sharp, and a long exposure in making a negative is re flu i red.
The invention of the camera obscura is of importance on account of its being the prototype of the modern photographic camera. and is claimed for a number of celebrated men. The first of these is Roger Bacon. who lived in the Thirteenth Century, and after him conies Alberti. about two centuries later. Leonardo da the famous painter and scientist, about the be ginning of the Sixteenth Century, says that if you place yourself in an hermetically closed room, facing a building, landscape, or any other object directly lighted by the sun, and then cut a small circular hole in the shutter, images of the objects outside will he thrown on any sur face facing the hole, and will he reversed. Such was the condition of the camera until the time of Cardan. about the middle of the Sixteenth Century, when he improved it by the addition of a convex lens at the aperture. The instru ment in this form is described in his work De ubtilitate. In 1569 Giambattista della Porta, of Naples. to whom the invention is generally as cribed, in the seventh book of his work on Natural Ilagic. gives a detailed description of the apparatus, and in his instrument he placed an inclined mirror before the lens, with the re sult that the pictures were rendered brighter and erect, instead of inverted. Porta's camera obscura attracted genera] attention, and the in strument was soon provided for the country houses of the wealthy, being placed usually in a small conical building, with a white table or surface in the centre, on which the pictures were projected. A more modern form is shown in Fig. 1.
The camera obscura in the form of a box, with inclined mirror and a piece of ground glass or paper on which the images could be traced, was invented by llooke in 1679. This instru
ment has since become familiar in the form of an optical toy, shown in diagram in Fig. 2, though the same principle has been made use of in certain pieces of scientific apparatus. The eamera obscura in its simple form of lens and screen, suitably incased, was first used for pho tographic work by Ilmnph•y Davy in 1802, at a time when lie was engaged in exile -nts with NVedgwood. After this, the camera ob scura was used by all the early experimenters in photography (q.v.), and the instrument has since been developed and amplified to a remark able degree. The photographic• camera is con strutted in an almost infinite variety of styles, depending in the main on the purpose for which it. is to be used. In general, it consists of two boxes, joined by a flexible and light-proof ma terial, such as a bellows of leather, rubber, or cloth, and susceptible of being moved toward or from each other, so that the distance be tween the lens and focal plane or position where the image is received can he varied. In one of these boxes, or on an upright board taking its place, the lens is mounted, and on the other there is a frame carrying a piece of ground glass, at the time of taking the picture, is removed and the sensitive plate or film sub stituted. This is contained in a holder so con structed that the sensitive surface occupies the same position as the original focusing glass.• The construction of a camera varies with the use or process for which it is intended. Thus, for out-of-door work portability and rapidity of adjustment are essential, whereas in a studio or laboratory rigidity and adaptability arc de manded. A view camera does not require any considerable length of bed, or base, as the ground glass is never distant from the lens a much greater amount than the principal focus (q.v.), as the pictures are generally on a reduced scale. If a camera, however, is designed for copying or enlarging. then it must have a long bed, as the distance front the lens to the ground glass will be as many times greater than the distance be tween the original object and the lens as it is de sired to enlarge the picture. Frequently in the ease of hand-eameras the ground glass is omitted, and the proper focus is obtained by reference to a graduated scale on the bed, on which are marked the foci corresponding to different distances.
The use of films and increased case of ma nipulation has made photography universally popular, and there are almost as many styles of cameras as there are individual tastes. See arti cle PHOTOGRAPI1Y.