LIGHT AND DISEASE 131 



ride, and hence appear dark on the negative. Dark objects, 

 such as furniture or a black coat, change little of the chlo- 

 ride, and hence appear light on the negative. To obtain a 

 true photograph, the negative is placed on a piece of sen- 

 sitive photographic paper, or paper coated with a silver salt 

 in the same manner as the plate and films. The combination 

 is exposed to the light. The dark portions of the negative 

 will act as obstructions to the passage of light, and but little 

 light will pass through that part of the negative to the photo- 

 graphic paper, and consequently but little of the silver salt 

 on the paper will be changed. On the other hand, the light 

 portion of the negative will allow free and easy passage of 

 the light rays, which will fall upon the photographic paper and 

 will change much more of the 

 silver. Thus it is that dark 

 places in the negative produce 

 light places in the positive 

 or real photograph (Fig. 84), 

 and that light places in the 

 negative produce dark places 

 in the positive ; all intermediate 

 grades are likewise represented 

 with their proper gradations of 

 intensity. 



If properly treated, a nega- 

 tive remains good for years, 



. . . .. , _ . FIG. 84. A positive or true photograph. 



and will serve for an indefinite 



number of positives or true photographs. 



125. Light and Disease. The far-reaching effect which 

 light has upon some inanimate objects, such as photographic 

 films and clothes, leads us to inquire into the relation which 

 exists between light and living things. We know from daily 

 observation that plants must have light in order to thrive 



