291 Wright— Optical Character of Birefr acting Minerals. 



the lines between the outer and inner circles represent the 

 actual position of the bisectrices and optic axes under the 

 conditions stated. 



Owing to the fact that for this section the angles of extinc- 

 tion are very low for all rays whose angle of incidence is small, 

 the intensity of the rays adjacent to those of the achromatic 

 curve is also low, since it varies with the square of the sine of 



Fig. 6. 



the angle p between the planes of polarization of the nicols and 

 that of the section according to the formula 



I 



sin 2 2 p sin 2 - (o — e) 



The black curves are therefore indistinct and require careful 

 scrutiny to be observed at all. 



The colored hyperbolic interference curves which appear in 

 the interference figure most sharply in the diagonal position of 

 the section can also be used to locate the direction of the acute 

 bisectrix. It can be proved in several different ways that the 

 acute bisectrix is generally direction of less birefringence than 

 the obtuse bisectrix. The birefringence of any section can 

 be figured approximately by the formula 



y — a' = (y — a) sin 6 1 sin # 2 



where y f and a' denote the maximum and minimum refractive 

 indices of the given section, 7 and a those of the mineral. 1 

 and # 2 the angles between the normal to the section and the 



