RELATIVE VALUE OF DIFFERENT KINDS OF GLASS 



301 



The equivalent areas required to give the same intensity of light 

 with the kinds of glass shown in Fig. 147, are given in Table XXIV 

 for sky angles of 30 and 60. 



TABLE XXIV. 

 EQUIVALENT AREAS FOR DIFFERENT KINDS OF GLASS. 



The American L,uxfer Prism Co., recommends that Luxfer prisms 

 be set at an angle of about 57 degrees with the vertical when used in 

 skylights. 



Relative Value of Different Kinds of Glass. Ground glass is of 

 little value except as a softening medium for bright sunlight. It be- 

 comes opaque with moisture and makes an undesirable window glass. 

 Roughened plate glass has very little value as a diffusing medium. Of 

 the ribbed glasses, the factory ribbed glass with 21 ribs to the inch gives 

 the widest and most uniform distribution and is distinctly the best. 

 There is no apparent gain in corrugating both sides. Ribbed wire glass 

 is about 20 per cent less effective than the factory ribbed glass. When 

 a glass of a slightly better appearance than the factory ribbed glass is 

 wanted the maze glass is the best ; the raised pattern imprinted on the 

 back of this glass giving wide diffusion, especially in bright sunlight. 

 The prisms are very much more effective than any of the glasses men- 

 tioned above, but their cost prevents their use under ordinary conditions. 



Kind of Glass to Use. Where the amount of skylight is large 

 and the light is not obstructed by buildings plane glass is very satisfac- 

 tory. Where a superior light is desired, or where the skylight area is 

 less than ample, use factory ribbed glass in skylights and in the upper 



