268 



GERTRUDE MAREAN WHITE. 



c 

 o 



10 



to 



i 



bO 



C 

 J 



c 



ID 

 O 



e 



500 600 



Wave-length y 



400 



FIG. 2. Per cent, of light transmitted by Wratten filters used to 

 secure monochromatic illumination. 



100 



