Transparent Inactive Crystal Plates. 211 



principle, however, the two methods are fundamentally differ- 

 ent, and a detailed discussion, together with a series of meas- 

 urements on interference figures of muscovite and anhydrate, 

 indicate the general validity of the principle on which the 

 method proposed by the writer is based ; in this method the 

 rotatory effects of all boundary surfaces are disregarded and for 

 this reason the results obtained by its use are only approxi- 

 mately correct. 



Several devices are described which have been found ser- 

 viceable in connection with this work: (1) An apparatus for 

 securing an intense and constant sodium light. (2) A simple 

 and accurate method for adjusting the petrographic micro- 

 scope. (3) A device to aid in the work with the stereo- 

 graphic projection plat. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 



Washington, D. C, November, 1910. 



