F. E. Wright — Measurement of Extinction Angles. 389 



the least satisfactory when accurate results are desired. It is 

 an exceedingly easy matter to measure, with one trial only and 

 on favorable sections, extinction angles with a probable error 

 of ±1° to 2°, but to do so within ±1° is a very different mat- 

 ter. In the foregoing pages the problem is first discussed 

 theoretically, and the mathematical equations covering the 

 different methods applied to extinction angle measurements 

 derived and discussed briefly. The methods for this purpose 

 may be grouped into two classes, — those of general application 

 and those of limited applicability. The first class may again 

 be subdivided into two classes ; either (1) the crystal is revolved 

 between crossed nicols about its position of true extinction, or 

 (2) the crystal remains stationary, and the accuracy of its position 

 of true extinction tested by revolving the upper nicol or by 

 inserting one of several different optical devices to increase the 

 sensitiveness of the test under the prescribed conditions of illu- 

 mination. These devices include the Calderon ocular, the 

 Bertrand ocular, the Bravais-Stober plate, the Traube plate, 

 also twinned plates and wedges of selenite, artificially twinned 

 plates and wedges of quartz, the circularly polarizing bi-quartz 

 wedge plate and the bi-nicol ocular, all of which are described 

 briefly above. Of these devices the last two are the most uni- 

 versal and can be so used under any given conditions of illu- 

 mination that the phenomena observed are the most sensitive 

 possible to attain by devices of this type. 



On comparing the relative sensitiveness of the different 

 methods under the same conditions, it is found that the method 

 of testing the position of true extinction for the crystal by rev- 

 olution of the upper nicol is, on colorless mineral-plates, at 

 least twice as sensitive as that of simply turning the crystal to its 

 position of apparent maximum darkness under crossed nicols.. 

 Under the same conditions the methods requiring the use of 

 one of the several plates or wedges mentioned above are at 

 least four times as sensitive as the ordinary method. With the 

 exception of the last two devices, however, these different 

 plates- do not furnish equally sensitive results for the different 

 conditions of illumination which may arise. In accurate work 

 adjustable sensibility is of prime importance, particularly if a 

 given device is to be of general application. These require- 

 ments are best filled by the bi-quartz wedge plate, by means of 

 which the angle of rotation can be varied from 0° to any desired 

 angle. The two halves of this wedge rotate in opposite direc- 

 tions, and on insertion that angle of rotation can be secured for 

 which the contrast in the intensity of the halves of the field is 

 most striking for a slight deviation of the crystal from its true 

 position of extinction. 



