LIGHT AND SHADE 265 



even coating. The sensitized surfaces are then dried in dark 

 rooms, and the plates are packed ready for sale. If you look 

 at a plate in the light, you see the yellow, creamy film of the 

 silver bromide emulsion. 



As glass plates are heavy and inconvenient, celluloid films 

 have almost entirely taken their place for outdoor work. If 

 plates or films coated with the silver bromide emulsion are 

 exposed in the camera and then carried back into the dark 

 room, one would expect to see an image. But we are much 

 disappointed because they show not the slightest change in 

 appearance. This is because the exposure was too brief to pro- 

 duce a change that the unaided eye can detect. The developer 

 must be used to bring out the hidden image. When the plate 

 has been in the developer for a few seconds, the coating gradually 

 darkens and the image slowly appears. When the image is fully 

 developed, the unchanged silver bromide is got rid of by fixing, 

 the plates are washed, and the negative is ready for use. 



Light and shade, Let us apply the above process to a 

 real photograph. Suppose we wish to take the photograph 

 of a man sitting in a chair in his library. If the man wore 

 a gray coat, a black tie, and a 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 bromide on the plate 

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

 image of the collar. The gray coat would not send to the 

 lens so much light as the white collar, hence the silver bromide 

 would be less affected by the light from the coat than by that 

 from the collar, and at the place where the lens produced an 

 image of the coat the silver bromide would not be changed so 

 much as where the collar image is. The light from the face 

 would produce a still different effect, since the light from the 



