236 IOWA ACADEMY OF SCIENCE 



lar arrangement ; and from the loeation of these spots with 

 reference to the cross-hairs, as photographed, the exact character 

 of the elliptic vibration can be readily calculated. 



In this particular application, the parallel beam of light is 

 reflected from a small metallic crystal and is very slender, so 

 that only a small portion of the field is illuminated at once. 

 In order to produce the spot pattern, the analyzing apparatus 

 is carried back and forth with a sort of weaving motion, at 

 right angles to the beam, until the whole field is covered. The 

 pattern then appears clearly on the plate, and measurements 

 are easily made upon it. Some excellent plates have been 

 obtained in this manner from very small crystals of selenium. 

 See figure 41. 



From such plates, it ought to be possible to settle the quastion 

 whether metallic crystals are doubly refracting. In fact, the 

 preliminary results would indicate that such is the case with 

 selenium. The research is still in progress. 



Coe College and 



State University of Iowa. 



