476 THE CAMERA OBSCURA. 



exposing the sensitized plate, a plate of ground glass 

 in order to adjust' the lens to the distance required for 

 obtaining a well-defined image upon the plate. The 

 photographic camera is usually a square box, often 

 made of two parts, one sliding in the other, so that the 

 whole may be either shortened or lengthened for the 

 purpose of adjustment. The front side carries a tube 

 in the middle in which the lens is fixed ; at the back 

 there is a frame into which the plate of ground-glass 

 and afterwards the sensitized plate may be placed. Be- 

 fore the latter is introduced the image is first rendered 

 as distinct as possible by directing the lens or ' objective' 

 towards the object, holding the head at a distance of 15 cm 

 or 20 cm behind the ground-glass plate, covering the 

 head, and the back of the camera, with a thick black cloth, 

 and then shortening or lengthening the camera until 

 a tolerably distinct image is seen on the plate; more 

 perfect definition is finally obtained by a finer motion 

 of the lens to or fro, which is given to it by a rack and 

 pinion. The objective of a camera does not in reality 

 consist of a single lens, but of a combination of several, 

 which acts in a like manner ; for the mere ^production 

 of a visible image, a single lens is sufficient, though not 

 for photographic purposes. 



On account of the inverted position of the real images 

 it is somewhat inconvenient to observe them ; by a 

 slight modification of the camera, the images may, how- 

 ever, be received on a horizontal surface, and thus 

 rendered more convenient either for observation or for 

 the purpose of copying them by drawing. Such a 

 camera is represented in fig. 274 ; it may easily be made 

 by the student with a little care. The rectangular box 



