228 ELEMENTARY SCIENCE 



fuse the result (spoil the picture), the construction of the 

 camera must be such that light enters it only from the 

 objects which are to be photographed (study Fig. 86). 

 The recording of the image depends upon the fact that 

 light is absorbed by the substance (usually silver chloride 

 or bromide) which coats the plate or film, and upon being 

 absorbed it decomposes this substance. Thus the image of 

 white objects appears black when the plate or film is de- 

 veloped, dark objects appear light, and gray effects are 

 produced by objects which were intermediate between 

 light and dark (colored). You can readily understand that 

 this is because the light reflected from light objects de- 

 stroys the chemical coating, while dark objects can produce 

 no such effect because they do not reflect light. Also you 

 can see why the developed plates or films are called nega- 

 tives. Positives, in which the normal appearance is re- 

 stored, are produced by placing the negatives over specially 

 prepared paper, and exposing both to sunlight. The 

 paper will then be altered as the negative was, but with 

 reverse effects. This process is called printing the picture. 



Candle-Power. The intensity or strength of light is 

 usually measured in terms of a unit called candle-power. 

 As a basis for this system of measuring there must be, of 

 course, a standard type of candle, uniform in all its details. 

 The candle-power of an electric lamp may be measured 

 as indicated in Fig. 87. Suppose the candle be set just 

 one foot from the upright rod. Then we will move the 

 lamp along line ab until we find the point at which the 

 shadow it casts on the screen is just equal in intensity 

 (darkness) with the shadow cast by the candle. Suppose 

 this point is just four feet from the rod. We know then 



