THE CAMERA 



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Essential parts of a plate camera. 



lens, and the distance from that point to the center of the 

 lens is called the focal length. You can easily measure the 

 focal length of a lens by this method. 



The camera. The study of a camera will help us to 

 understand the parts of the eye. If we carefully note the 

 parts of the camera 

 shown in the illus- 

 tration, we find that 

 it is a light-tight box 

 painted black on the 

 inside to absorb all 

 light rays, and hav- 

 ing a lens, a dia- 

 phragm for changing 

 the size of the open- 

 ing, and a shutter in 

 front which controls the time of the exposure of the photo- 

 graphic plate or film which is placed at the opposite end 

 of the camera. In a photographer's camera, such as is 

 shown in the illustration, a ground glass is placed at the 

 end where the plate is held when an exposure is made. 



Experiment. To show how a lens is used in focusing 



Materials : Double convex lens. White screen. Large carbon bulb. 



1. Focal length. 



Hold a double convex lens in sunlight at right angles to the rays. 

 Place a screen of white paper back of the lens. Adjust the distance be- 

 tween them until the spot of light (the sun's image) is the smallest. 

 Measure the distance from the lens to the screen. This distance is 

 called the focal length of the lens. 



Compare the focal length of a thick lens with that of a thin lens by 

 this method. 



2. Focusing as done in the camera. 



Hold a double convex lens about three times its focal length from 

 a large carbon lamp which is lighted in a dark room. Move a screen of. 



