37-i F. E, Wright — Measurement of Extinction Angles. 



the angle but also on the different compensations of the path 

 differences in the two plates, and if white light be used this 

 results in a rapid change in interference color in the two 

 halves if the crystal be only a small angular distance from its 

 position of true extinction. 



The writer has had such a plate cut showing the sensitive 

 tint and also a wedge, so that on insertion different interference 

 colors, or intensities in monochromatic light, can be used for 

 which the eye under certain conditions is most sensitive. These 

 plates, as w T ell as the preceding, are inserted in the focal plane 

 of the objective and the junction line serves for the vertical 

 crosshair. For such plates the Kamsden positive ocular has 

 been found by experience to be best suited and a specially con- 

 structed holder convenient. 



Artificially twinned Quartz plate. — Still another advan- 

 tageous arrangement can be had by cutting on a polished quartz 

 plate parallel to the principal axis a vertical edge making an 

 angle of about 3°-6° with the principal axis. The quartz 

 plate is then divided transversely to the polished edge and 

 the polished edges cemented together, thus producing an arti- 

 ficial tw T in whose two halves extinguish at equal and oppos- 

 ite angles from the common line of junction. Such plates 

 may then be ground to a thin plate showing either the sensitive 

 tint or dull gray of the first order or to wedge form, thus increas- 

 ing the range and usefulness of the device. 



All of the preceding plates, the Bravais-Stober, the Calderon 

 and the Traube, the selenite twin plate and the quartz combina- 

 tion plate wedges of the last paragraph, can be made somewhat 

 more sensitive by dividing the field into quadrants instead of 

 halves, after the example of Bertrand in his rotatory polarizing 

 quartz plates described below. 



Bi-nicol ocular. — In the practical application of these differ- 

 ent types of plates the angle 6 has been small (2 — 4°) and 

 found to furnish good results, but in each case there is a par- 

 ticular angle 6 which is best adapted for the observations ; the 

 limit of sensitiveness of different eyes introduces, moreover, a 

 variable element of such wide range that the angle 6 cannot be 

 calculated and fixed once for all. In order therefore to have 

 control over all angles 6 and thus in each instance to be in 

 position to procure the best possible conditions, the writer has 

 had constructed the following ocular attachment. 



The principle of construction of the apparatus is apparent 

 from the figure, and need not be expressed at length at this 

 point. The light after passing through the lower nicol and 

 the crystal plate reaches the lower reflecting prism pair of this 

 ocular and passes thence through appropriate nicol prisms 

 (Thompson prisms) or birefracting plates of exactly the same 



