62 



THE PRODUCTION AND USE OF LIGHT 



in libraries, and even in factories, are very generally so con- 

 structed as to give only diffused light. This is especially true 

 of gas and electric lighting. The gas mantle is often sur- 



FIG. 48. Diffused light 

 from the frosted bowl of a 

 tungsten lamp. 



FIG. 49. Diffused light from the opal 

 glass globe of the gas mantle lamp. 



rounded by a suitable opal glass globe; the lower portion of 

 the electric light globe is frosted, while the upper portion is 

 surrounded by a reflector which diffuses the light. Figure 48 



shows how the direct, parallel 

 rays from the tungsten light are 

 diffused by the frosted bowl, and 

 Fig. 49 shows how the direct rays 

 from the gas mantle are diffused 

 by the white glass globe. In 

 each case, the pencil of parallel 

 rays A is broken up into the 

 diffused rays, a, a, a, and a, while 

 the pencil of parallel rays B is 

 broken up into the diffused rays b, 6, b, and b. On the other 

 hand the direct rays C pass through the clear glass of the 



FIG. 50. A fluted glass reflector. 

 Used on gas and electric lamps. 



