LIGHT AND SHADE 1 29 



Light from the object to be photographed passes through 

 the lens, falls upon the screen, and forms an image there. 

 If we substitute for the ordinary screen a plate or film 

 coated with silver chloride or any other silver salt, the 

 light which falls upon the sensitive plate and forms an image 

 there will change the silver chloride and produce a hidden 

 image. If the plate is then removed from the camera in the 

 dark, and is treated as described in the preceding Section, the 

 image becomes visible and permanent. In practice some 

 gelatin is mixed with the silver salt, and the mixture is then 

 poured over the plate or film in. such a way that a thin, even 

 coating is made. It is the presence of the gelatin that gives 

 plates a yellowish hue. The sensitive plates are left to dry 

 in dark rooms, and when the coating has become absolutely 

 firm and dry, the plates are packed in boxes and sent forth 

 for sale. 



Glass plates are heavy and inconvenient to carry, so that 

 celluloid films have almost entirely taken their place, at least 

 for outdoor work. 



123. Light and Shade. Let us apply the above process to 

 a real photograph. Suppose we wish to take the photograph 

 of a man under the historic trees of Mt. Vernon. If the man 

 wore gray trousers, a black coat, and white collar, these details 

 must be faithfully represented in the photograph. How can 

 the almost innumerable lights and shades be produced on the 

 plate ? 



The white collar would send through the lens the most light 

 to the sensitive plate ; hence the silver chloride on the plate would 

 be most changed at the place where the lens formed an image 

 of the collar. Gray trousers would not send to the lens so much 

 light as the white collar, hence the silver chloride would be less 

 affected by the light from the trousers than by that from the 

 collar, and at the place where the lens produced an image of 



CL. GEN. SCJ. Q 



