388 Wright — Two Microscopic- Petrographical Methods. 



are those cut perpendicular to the three ellipsoidal axes a, b, c, 

 and the two optic axes (optic binomials) ; in the uniaxial min- 

 erals these quantities reduce to plates cut parallel and perpen- 

 dicular to the principal axis. 



By observing the interference figures on plates perpendicu- 

 lar to the bisectrices a or c of biaxial minerals, we are able 

 to separate the biaxial minerals from the uniaxial. The optical 

 character of the uniaxial minerals whether positive or negative 

 can always be determined, while in the biaxial minerals this can 

 only be effected when the angle between the optic axes in air 

 (2 E) is small (less than 90°) — when both optic axes appear in 

 the field, the angle of which is 70°-90° in ordinary micro- 

 scopes. For the larger part of biaxial minerals, however, 2 E 

 is greater than 90° and cannot be used as a general factor in 

 the practical separation of minerals by ordinary means. The 

 same holds true for plates cut perpendicular to the optic axes. 

 For this reason the optical character of biaxial minerals is rarely 

 applied in their microscopic diagnosis. 



By using the interference phenomena, however, observed in 

 convergent polarized light on plates perpendicular to the optic 

 normal (b ellipsoidal axis in the biaxial minerals, and the nor- 

 mal to the principal axis in the uniaxial minerals), the optical 

 character of the rock-forming minerals can be ascertained 

 by ordinary means. 



A plate perpendicular to the optic normal (either uniaxial or 

 biaxial) Exhibits a peculiar interference figure in convergent 

 polarized light. On revolving the stage of the microscope, the 

 field becomes dark suddenly, remains so for an instant, only to 

 become light again on further revolution through a few 

 degrees. In the position of darkness the ellipsoidal axes a and 

 b are parallel to the plane of vibration of the nicols. No 

 distinct cross is seen as in the interference figures of plates 

 perpendicular to the bisectrices. The entire field appears dark, 

 with perhaps a weak fringe of light along the outer extremities 

 of the diagonals of the quadrants. . 



If the field be placed in the dark position and turned slightly, 

 faint, dark hyperbolas can be seen to open and leave the cen- 

 ter of the field, similar to the dark hyperbolas of the biaxial 

 interference figures perpendicular to the bisectrices, the chief 

 difference between the two being one of intensity and rapidity 

 of motion. The hyberbolas are very weak and require close 

 observation to be noticed at all. A mineral plate of low bire- 

 fraction will show them less distinctly than one of high. 



Rule. — The dark hyperbolas leave the field in the direction 

 of the acute bisectrix in the biaxial minerals and in the direc- 

 tion of the principal axis in the uniaxial minerals. 



Application. — Observe direction in which the faint hyper- 



