380 



Messrs. Nichols and Snow on the Selective 



merely relative. Expressed graphically, the curve of a 

 substance the absorbing power of which is not selective is a 

 horizontal line, with ordinate equal to unity. 



Figure showing selective absorption : — (A) by a glass lens ; 

 (B) by a pair of Nicol's prisms. 



•V 



.6* 



-.*/• 



Tables I. and II. show the selective transmission in glass 

 and calcite, respectively, expressed in che manner already 

 indicated. Curves A and B of the accompanying figure 

 present the data of the tables in graphic form. Abscissae 

 are wave-lengths and ordinates represent amounts of light 

 transmitted. 



