304 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



IV. THE OPTICAL APPARATUS EMPLOYED IN PHOTOGRAPHY. 



Now that we have given an idea of the principal methods of 

 photograhpy which have succeeded each other since the invention of 

 Niepce and Daguerre, it will be well to revert to a point common 

 to all, and to enter into some details on the optical apparatus 

 namely, on the arrangement of the camera obscura with its most 

 important accessories. 



The camera obscura, ordinarily called a. camera, is in its simplest 

 form composed of a rectangular wooden box, formed of two or more 

 compartments resting on a sliding board. This enables the box to 



FIG. 218. Photographic cameVa. 



be lengthened or diminished at will in one direction. Cameras are 

 now frequently made on the principle of bellows, avoiding the 

 necessity of the different compartments of the box. 



In front is an opening, AB, carrying a tube, holding the object- 

 glass. In this are arranged fhe glasses or lenses destined to produce 

 the image of the objects to be photographed. The back of the 

 camera is arranged to receive in a groove the frame G, which holds 

 the sensitive plate on which the light is to impress the image. 

 Before admitting the light, however, to the sensitized surface, it must 

 be ascertained that the image is well in focus. This the operator 



