146 HOW WE USE LIGHT 



opened much wider and still give a sharp image. This 

 lets more light in and shortens the exposure, and the lens 

 is called "rapid." The cheap lens may give just as good 

 a picture if you can give it sufficient time with the small 

 opening. 



Making a Negative. Many of you take pictures but 

 give the films to a photographer to develop. If you do 

 this, you lose half the fun. Why not learn to develop the 

 films yourself? Most junior high schools have camera 

 clubs and dark rooms, so you will have no difficulty in 

 doing your own work. The plate or film is coated with 

 gelatin containing silver bromide, a substance sensitive to 

 light. When light from an object in front of the lens is 

 focused on the film, it makes an image upon it which does 

 not become visible until after the film is developed. De- 

 velopment takes place in a dark room which has only a 

 red or ruby light in it. This light does not act upon the 

 film during the short time required to change it into a 

 negative. The film is first treated with a "developer," 

 a chemical solution which causes a deposit of dark particles 

 on the film. No dark deposit is made on those parts of 

 the film not reached by the light. The differences of 

 light and dark at any point is in proportion to the amount 

 of light which acted upon the silver bromide. When 

 development is complete, some silver bromide which was 

 not acted upon by the light and which is still sensitive to 

 it remains on the film. Before the film can be exposed 

 safely to ordinary light, this silver bromide must be re- 

 moved. A chemical salt, hyposulphite of soda, called 

 "hypo," will dissolve the silver bromide and remove it 

 from the film. This is used as a "fixing bath," because 

 it makes the image permanent. After the film has been 

 fixed, washed, and dried, the light and dark areas in it are 

 just the reverse of those in the original picture and for 

 this reason it is called a negative. 



