264 PHOTOGRAPHY 



light without fear. To accomplish this, the unchanged silvef 

 chloride is got rid of by " fixing," that is, by washing off the 

 unreduced silver chloride with a solution known as " hypo/' 

 After a bath in the hypo the plate is cleansed in clear running 

 water and left to dry. Such a process gives a clear and per- 

 manent picture on the plate. 



The camera. A camera (Fig. 134) is a light-tight box con- 

 taining a movable convex lens at one end and a screen at the 



opposite end. 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 

 screen a plate coated 



with silver chloride, the 



FIG. I34.-A camera. 



sensitive plate will slowly change the silver chloride. If we 

 give a sufficiently long exposure to the light an image will be 

 formed. But this exposure would be too long to be practical, 

 and so we must help the action of light by a chemical process. 

 Certain substances called " developers " separate the silver from 

 the chlorine in silver chloride. Therefore, instead of giving an 

 exposure long enough to allow the light to complete its work on 

 the silver chloride we give an exposure just long enough to 

 allow the light to start the chemical action. Then the plate 

 is placed in a developer, which quickly completes the liberation 

 begun by the sunlight and produces the image. Silver bromide 

 is more easily acted upon by developers than silver chloride 

 and is now generally used for plates. Silver bromide is pre- 

 cipitated in gelatine with which it forms an emulsion. This 

 silver bromide emulsion is spread over glass plates in a thin 



