342 



merwin: chromatic reflection in covellite 



for deterniiiiations of refractive index. ^ 

 values are plotted in figure 1 : 



The following observed 



Different directions in the plane of the plate give the same value. 

 Between crossed nicols, flat-lying plates appear dark and ex- 

 hibit no distinct interference figure. Yet tilted plates are 



a doubly re- 

 fracting and 

 pleochroic. 

 One raj' vi- 

 brates in the 

 plane of the 

 plates and is 

 darker for all 

 colors. For 

 both rays 

 there appear 

 to be regions 

 of much 

 stronger ab- 

 sorption at both ends of the visible spectrum. These are closer 

 together for the ray vibrating in the plane of the plates. 



* Practically opaque plates, measured with the micrometer caliper, were 

 only 0.002 mm. thick. Moderately transparent plates were so thin that the 

 displacement of the bright border of light produced in determining the refractiA^e 

 index was so slight that high magnification and a very perfect fine adjustment 

 screw were required to make the displacement e\adent. Even then differences 

 of less than ±0.03 in refractive index could not be detected, owing partly to 

 diffraction. In many plates the diffraction bands entirely obscured the phenom- 

 ena of refraction. The method finally pursued involved the use of a monochro- 

 matic illuminator. Only such plates were used as showed a pronounced reversal 

 of the position of the bright border for wave-lengths about 10 jx greater or less 

 than that for which the determination of refractive index was made. 



Fig. 1 



