128 



PHOTOGRAPHY 



cal, the so-called "developer," must be used to bring out the 

 hidden change and to reveal the image to our unseeing eyes. 

 There are many different developers in use, any one of which 

 will effect the necessary transformation. When the plate has 

 been in the developer for a few seconds, the silver coating 

 gradually darkens, and slowly but surely the image printed by 

 the sun's rays appears. But we must not take this picture 

 into the light, because the silver chloride which was protected 

 by the horseshoe is still present, and would be strongly 

 affected by the first glimmer of light, and, as a result, our en- 

 tire plate would become similar in character and there would 

 be no contrast to give an image of the horseshoe on the plate. 

 But a photograph on glass, which must be carefully shielded 

 from the light and admired only in the dark room, would be 

 neither pleasurable nor practical. If there were some way by 

 which the hitherto unaffected silver chloride could be totally 

 removed, it would be possible to take the plate into any light 

 without fear. To accomplish this, the unchanged silver 

 chloride is got rid of by the process technically called " fix- 

 ing"; that is, by washing off the unreduced silver chloride 



with a solution such 

 as sodium thiosulphite, 

 commonly known as 

 hypo. After a bath in 

 the hypo the plate is 

 cleansed in clear run- 

 ning water and left to 

 dry. Such a process 

 gives a clear and perma- 

 nent picture on the plate. 

 122. The Camera. A 



camera (Fig. 82) is a light-tight box containing a movable 

 convex lens at one end and a screen at the opposite end. 



FIG. 82. A camera. 



