384 F. E. Wright — Measurement of Extinction An^ 



tes. 



angle <£ can be varied at will, This is true of both the circu- 

 larly polarizing wedge plate and the bi-nicol ocular, and by 

 their use the probable error of the extinction position of any 

 crystal plate is at least one-fourth that of a determination 

 after the usual method by revolving the crystal plate under 

 crossed nicols. Experience has shown that with favorable 

 sections, extinction angles can be determined by the use of the 

 bi-quartz wedge with a probable error of less than ±10' on a 

 single trial. 



Still another method for obtaining the most favorable con- 

 ditions of experiment with a given plate is that suggested on 

 page 374 with the artificially twinned quartz wedge. The two 

 halves of this wedge extinguish at a small angle (as 3°) on oppo- 

 site sides of the line of junction, and by inserting the wedge 

 that particular interference color, or phase difference if mono- 

 chromatic light be employed, can be produced for which the 

 given angle of revolution (3°) is the best. This wedge, how- 

 ever, is less favorable than the circularly polarizing bi-quartz 

 wedge, since its twinning line must be inserted precisely 

 parallel with the plane of the polarizer, while with the circu- 

 larly polarizing bi-quartz wedge the rotation of the planes of 

 polarization of transmitted waves is entirely independent of 

 the line of junction of the adjacent halves. 



In the preceding pages, special emphasis has been placed on 

 those methods for measuring extinction angles which are of 

 general application and which are based on intensity differ- 

 ences. The other methods, which are of limited application, 

 and can be used only in white light on favorable sections, 

 depend on differences in interference colors produced by slight 

 deviations of the crystal plate from its position of true extinc- 

 tion. Although these methods are serviceable in many 

 instances, their application and the results obtained thereby 

 are so dependent on the conditions of experiment that they are 

 difficult to treat satisfactorily in a general way. Experience 

 has shown that they are not more sensitive than the other 

 methods and usually much less so. This is true both of the 

 selenite sensitive tint plate and of all combinations of the same. 



Experimental Tests. — To test the different methods under 

 different conditions, different mineral plates were chosen and 

 the position of true extinction on each determined by the dif- 

 ferent methods under precisely the same conditions of illumin- 

 ation with white light. — On an anhydrite plate showing white 

 interference tints of the higher orders the possible error of a 

 single determination by revolving the crystal plate under 

 crossed nicols was found to be about 1;1° ; by revolving the 

 upper nicol alone, '4° ; by inserting the quartz wedge, about 

 0'1° ; by using the Calderon ocular, about # 5° ; by means of 



