Transparent Inactive Crystal Plates. 



199 



if Z'Y' (tig. 10) be the extinguishing plane of the analyzer, and 

 H any dark point in the field of the interference figure, the 

 direction of vibration for this light wave H must be contained 

 in the plane Z'Y' and also in the polar plane to H/; it is accord- 

 ingly the direction D. If its direction of vibration be not in 



the plane Y'Z', it will not be totally extinguished in the upper 

 nicol and the point H will not appear completely dark. Briefly 

 stated, for any dark point H of the interference figure, the 

 direction of vibration is the intersection OD of the extinguish- 

 ing plane X'Z' of the upper nicol and the polar plane of H. 

 This is the rule of construction given by the writer for finding 

 the plane of vibration of any dark point in the interference 



