LIGHT AND DISEASE 



267 



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. But little light will pass through that part 

 of the negative to the photographic 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 fall upon the photo- 

 graphic 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 photo- 

 graph (Fig. 136), and that light 

 places in the negative produce 

 dark places in the positive; all 

 intermediate grades are like- 

 wise represented with their 

 proper gradations of intensity. 



If properly treated, a nega- 

 tive remains good for years, 

 and serves for an indefinite 

 number of positives or true 

 photographs. 



Light and disease. The far-reaching 

 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 obser- 

 vation that plants must have light in order to thrive and grow. 

 A healthy plant brought into a dark room soon loses its vigor 

 and freshness, and becomes yellow and drooping. Plants do 

 not all agree as to the amount of light they require, for some, 

 like the fern and the arbutus, grow best in moderate light, 

 while others, like Indian corn, need the strong, full beams of the 



FIG. 136. A positive or true photograph. 



effect that light 



