189G.] 



NEW YORK ACADEMY OF SCIENCES. 



53 



reference to the plane of the nicols {i. e., extinction directions 

 at 45° to nicols) and the horizontal axis is diagonally placed. 

 Revolve the carrier about the horizontal axis, and the polariza- 

 tion colors will fade to approximate darkness at the emergence 

 of an optic axis and again ascend on further revolution. 



2 ° Character of double refraction in sections approximatehj 



normal if the middle axis of elasticity i. e., a2oproxi- 



mately parallel to the axial plane. 



The method of v. Federow * may be used. With the carrier 

 horizontal find the extinction corresponding to say rt, and read 



it on the rim T^, then with C parallel to the horizonal axis revolve 

 the carrier around this axis until nearly vertical, then turn the 

 carrier in its own plane until the brightening of the field shows 

 the emergence of an optic axis. The angle between this posi- 

 tion and that at first reading is the angle between <l and the op- 

 tic axis, if this is less than 45° a is the acute bisectrix and the 



crystal is negative, if the angle is greater than 45' 

 bisectrix and the crystal is positive. 



C is the acute 



3° Character of double refraction in sections approximately 

 normal either to the axes of greatest or least elasticity d or (. 



In sections approximately normal to (t, with the carrier hori- 

 zontal, the nicols crossed and C at 45° to the vibration direction 

 of the nicols, insert a gypsum red of the first order with its axis 



* Zeit. /. Kryst. 1895 v. 24 p. 604. 



