GENERATION, CONTROL, AND MEASUREMENT 215 



films containing graphite particles in suspension all decrease in trans- 

 mittance with decreasing wave length owing to the small size of the 

 absorbing particles that produce Rayleigh scattering. These should be 

 caUbrated separately at each wave length and under the conditions of use, 

 since they are never completely neutral over a wide range of wave lengths. 



INVERSE-SQUARE LAW^ 



Application of the inverse-square law is a useful means for predicting 

 relative irradiances over large values, provided that the source and object 

 to be irradiated are small compared with the distance between the two. 

 Not only must the source be relatively small, but also extraneous reflec- 

 tions from surrounding objects must be eliminated, since they can con- 

 tribute appreciable additional irradiance. If the surface of the irradi- 

 ated object is not normal to the direct rays from the source, the Lambert 

 cosine law must be applied. 



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