118 PROCEEDINGS OF THE AMEBICAN ACADEMY. 



be centred within i° accuracy. The plate was immersed in monobrom- 

 oaapthalin whose temperature during the measurement was 19° and 

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

 meter ;is : n na = 1.6597. With the thin section the following values 

 Were determined in the axial angle apparatus : 



whence, as w^ for the section was found to be G2 5 57' 15" and (3= 1.6842 : 

 2 V a for sodium = GO" 0' and p > v. 

 Munich, April, 1900. 



