316 OUR PHYSICAL WORLD 



the half still covered, out of the darkroom and put it in strong 

 sunlight. Very shortly the uncovered portion turns dark, in 

 time black, but the covered portion remains yellowish white. 

 When the plate or film is exposed in the camera the light 

 areas of the image, such as those of the cuffs or shirt bosom 

 in the image of a man, are affected by the light while the dark 

 areas, such as the image of a black coat, remain largely unaffected. 

 No image is visible on the plate, however, as the exposure is so 



FIG. 1 60. Diagram of a reflecting camera 



very brief. The latent image is brought out only when the plate 

 is chemically treated by the developer. 



The sensitive plates, films, and sensitized paper that the 

 photographer handles in order to make his negatives and print 

 his pictures must be handled in light that will not affect these 

 objects. As a matter of fact, all light does not affect them 

 equally, but the rays that are at the violet end of the spectrum 

 are the most active ones. By covering the ordinary sources 

 of light, the window or electric light, with screens of orange and 



