380 F. E. Wright — Measurement of Extinction Angles. 



no adjustment and of not suffering from the false light of 

 depolarization noted above. 



Convergent polarized light. — Two methods have been pro- 

 posed which require convergent polarized light and are based 

 on the change in aspect of symmetrical interference figures 

 caused by the intervening crystal plate when it is not pre- 

 cisely in the position of zero extinction. The idea underlying 

 the methods is that the eye can detect more readily slight 

 changes in the shape of a symmetrical interference figure 

 than proportionate changes in intensity or color. Theoret- 

 ically, this principle is excellent, but its practical application 

 to mineral sections is less satisfactory. The first method of 

 this type was proposed by Kobell in 1851, who used a plate of 

 calcite normal to the optic axis as his test plate. The micro- 

 scope was arranged for convergent polarized light and the crys- 

 tal plate with the calcite test plate above it placed on the 

 microscope stage and .turned until the interference figure 

 appeared perfectly normal and unclistorted. Practically, the 

 following objections apply to this method. The optical system 

 of the miscroscope requires changing each time to meet the 

 new conditions ; during the observations the crystal itself is 

 lost sight of, and in the case of minute crystals or crystals with 

 undulatory extinction this is a serious drawback. Moreover 

 it is tacitly assumed that in the crystal plate itself for direc- 

 tions other than the normal to its surface of the crystal plate 

 the planes of polarization remain parallel, which in general is 

 only approximately true even for small fields which include 

 only a small angle with the normal. 



In the Brezinaf method a more complicated interference 

 figure is produced by two calcite plates cut at a small angle 

 with the optic axis and cemented together one above the other 

 in such a way that the optic axes of the two are in the same 

 plane and at equal angles with the normal. The interference 

 figure from such a combination is noteworthy because of a 

 dark vertical bar through the center of the field. A slight 

 revolution of an intervening crystal plate displaces this bar 

 noticeably, but the same objections noted in the Kobell method 

 apply with equal force to this method, with the result that 

 neither method is made use of at the present time by working 

 petrologists. In fact, both these methods were suggested 

 before the petrographic microscope had been introduced. 



The relative sensitiveness of the different methods. — The 

 term position of extinction means practically that position of a 

 birefracting plate for which light waves are transmitted with- 

 out changing their plane of polarization and for which no light 



*Pogg. Ann., xcv, 320, 1855. 



f Described in Schrauf's Lehrb. d. Phys. Mm. ii, 219-220, 1868. 



