510 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the case ; the slope of the wedge, however, is not exactly correct, and a 

 slight correction must be applied to the readings obtained, since 22 mm. 

 on the wedge is equivalent to 22 " 62 /*/*. For interference colours of 

 the first and second order this error (nearly 3 p.c.) is very slight and can 

 practically be neglected ; but for higher orders it must be taken into 

 account and the readings multiplied by a factor of proportion. In 

 determining the birefringences (y - a), or (y - (3, or ft - a) of a mineral, 

 the position of the mineral plate (under examination) is ascertained by 

 means of convergent polarized light. In actual work it is not always 

 easy to find a plate cut precisely normal either to the optic normal or to 

 one of the bisectrices, and it is of interest to know the percentage error 

 •caused by using sections inclined at low angles with the correct directions. 



Fig. 62. 



For a given plate the birefringence can be calculated approximately 

 from the usual formula 



(■/ - a') (y - a) = sill I' sill I, 



in which I and I' are the angles whicli the normal to the plate makes 

 with the two optic axes (or optic binomials) respectively. In figs. 62-67 

 these relations are shown graphically in stereographic projection. In 

 each figure the angular distance between any two successive concentric 

 circles is 10°. In fig. 62 the positions of the directions in a biaxial 

 crystal whose birefringence (-/ - a') is 2 p.c. less than that of the optic 

 normal (y - a), are indicated for the optic axial angles 2 v = 0°, 45°, and 

 90°. The optic normal coincides with the central point of the figure. 

 Fig. 63 shows positions of the directions for which the birefringence 

 (y' - a') is 5 p.c. less than that of the optic normal (y - a) which coin- 

 cides with the centre of the concentric 10° circles. These curves are 

 drawn corresponding to the optic axial angles 2v = 0°, 45% and !)0°. 



