8 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



refraction placed between them is without effect on this extinction 

 of the light and is said to be isotropic. But a substance possessing 

 double refraction will, in all but one or two directions, cause light to 

 be transmitted and is then described as anisotropic. The latter is 

 also subdivided on the basis of number of optic axes (directions along 

 which polarized light is not affected), crystals derived from revolution- 

 ellipsoids having one such axis and being termed uniaxial, while 

 those not so derivable have two and are termed biaxial. There is 

 still another feature which is usually stated in connection with aniso- 

 tropic crystals — the optical sign. When the greatest index of refrac- 

 tion is shown in the direction of the optic axis, in uniaxial crystals, 

 or in the acute angle between the two optic axes in biaxial ones, 

 the sign is described as plus ( + ) ; and in reverse case it is minus ( - ). 



