Chap. xxix. j THE CAMERA OBSCURA. 375 



camera obscura is well known in its form of photo- 

 graphic camera. It consists of a box (Fig. 169), black- 

 ened in the interior to prevent reflection from the walls. 

 In front is a short tube hi containing a system of 

 achromatic lenses. For the back wall of the box is sub- 

 stituted a ground-glass plate g, 

 on which the image formed by 

 the lens is focussed. In photo- 

 graphy, for the ground-glass 

 plate a plate sensitive to light is 

 substituted, on which the imaoje 



,-1 mi v i i Fig. 169. Camera Obscura 



is thrown. I he action or light 



on the sensitive surface of the plate produces 

 chemical changes, varying in degree according to the 

 varying intensity of the light in different parts of the 

 image. So that on developing the image by various 

 solutions, the salts of the sensitive coating, that have 

 been acted on by the light, are deposited on the plate. 

 At a point of the image corresponding to a point of 

 the object from which no light was reflected to the 

 camera, no change will have occurred, and that part of 

 the sensitive plate will be removed from the plate. 

 Thus grades of thickness in the plate's coating will be 

 produced, according to the varying lights and shades of 

 the object, and these will constitute the developed 

 image. Besides dark chamber, lens, and sensitive 

 plate, other arrangements are necessary. If the 

 camera be so adapted that parallel rays falling on the 

 lens are brought to a focus on the sensitive plate, it is 

 obvious that divergent rays will not be focussed on. the 

 plate, but behind the plate, so that a blurred instead 

 of a sharp image would result. If, however, the 

 sensitive plate could be moved backwards, it could be 

 made to coincide with the conjugate focus of the rays 

 diverging from the object. This is effected by making 

 the chamber in two halves (b and a), one telescoping into 

 ths other, so that the chamber can be lengthened ov 



