118 PROCEEDINGS OF THE AMERICAN ACADEMY. 



be centred withiu J° accuracy. The plate was immersed in monobrom- 

 ouapthalin whose temperature during the measurement was 19° and 

 whose index of refraction was determined by the Abbe total-refracto- 

 meter as: n^^^ = 1.6597. AVith the thin section the following values 

 were determined in the axial angle apparatus : 



whence, as to^ for the section was found to be 62^ 57' 45" and li= 1.6842 : 

 2 Va for sodium = 60° 0' and p > v. 

 Munich, April, 1900. 



