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



plates in combination instead of a single one. A single sensi- 

 tive tint plate of mica or quartz is taken and cut along a line 

 at -±5° with the directions of extinction ; the one half is then 

 turned through angle of 180° and the two halves recemented 

 as indicated in the figure. By this combination plate, which is 

 placed in the focal plane of the ocular, the interference color 

 is made to fall in the one half and to rise an equal amount in 

 the second, thus doubling the sensitiveness of the single plate. 



Fig. 8. 



This plate is intended for use only in white light, but under cer- 

 tain, conditions it may serve to good advantage in monochro- 

 matic light. 



The Combination Wedge* — On the principle of the Bravais- 

 Stober plate, the writer has had a combination wedge prepared 

 in which the interference colors range from total darkness to 

 green of the second order. This wedge was made by taking 



*This wedge was prepared with great care by Voigt & Hochgesang, 

 Gottingen, Germany (cost, 48 mks.), and the writer desires to express his 

 appreciation of the interest taken by the firm in the same. The compensation 

 on different ends of the wedge, however, proved to be of unequal value, with 

 the result that although the dark zero interference bands were precisely 

 adjacent, the interference colors near the ends of the wedge did not coincide 

 exactly. This defect could be eliminated by combining two quartz plates 

 (45 mm long by 5 mm wide and of such a thickness as to show interference color 

 green-yellow second order, the ellipsoidal axis c of the one to be parallel to 

 the long direction, while in the second a is parallel), with two wedges of the 

 same pitch (45 mm long by 5 mm wide and ranging in interference colors pale 

 gray of the first order to yiolet gray at the top of the third order, and like- 

 wise the ellipsoidal axis c parallel to the direction of elongation in one and a 

 to the same direction in the second) ; the wedge of long direction c to be com- 

 bined with the plate of long direction fl. In this manner the plate and wedge 

 compensate in the center of the wedge and the interference colors rise to about 

 blue green of the second order at both ends. 



