F. E. Wright — Measurement of Extinction Angles. 385 



the Bertrand ocular, about 0'1° ; with such a plate the sensi- 

 tive tint plate is of no value since the interference color of the 

 anhydrite plate itself is so high that the violet of the inserted 

 plate has no effect and the differences in intensity which occur, 

 do so in a strongly lighted field and are not easily discernible. — 

 Similar measurements were made on an apatite plate parallel 

 to 1010 and showing the interference tint, red of the first 

 order. The possible error of a single determination of the 

 position of true extinction on turning the crystal plate alone 

 under crossed nicols was found to be ±'9° ; on revolving upper 

 nicol about -2°, accurate because of abnormal interference 

 colors which appear when the plate is distant only a slight 

 distance from its correct extinction position ; on inserting the 

 bi-quartz wedge plate 0'2° to 0'3° ; with the Calderon ocular, 

 about 0*3° : with the Bertrand ocular, about 0*3° ; the sensi- 

 tive tint plate is again of no value since the interference color 

 changes comparatively slbwly_as crystal is revolved. — A sec- 

 tion of nephelite parallel to 1010 and showing the interference 

 color, yellow first order, gave the following results : On 

 revolving the crystal plate alone, possible error '±° ; on rotat- 

 ing upper nicol, less than 0*1 ° ; with bi-quartz wedge less than 

 0*1° ; Calderon ocular about 02° ; Bertrand ocular less than 

 0*1°; sensitive violet plate still of very little value as a 

 method; only slight changes in color for large angles of 

 revolution of plate. On a plate of colorless gehlenite of very 

 low interference color, dull gray, first order, the sensitive tint 

 plate proved as satisfactory as any other and more so than the 

 method of turning the crystal plate under crossed nicols or of 

 revolving the upper nicol or the Calderon ocular. The Ber- 

 trand ocular and the bi-quartz wedge plate proved about as 

 favorable, the probable error being slightly less than 0'5°. — A 

 plate of strongly pleochroic tourmaline was also used and the 

 following results obtained : Probable error of determination 

 on revolving crystal plate alone, about l - 6° ; the method of 

 revolving upper nicol is of little value because of deep natural 

 color of mineral and consequent inability to match fields ; 

 with the bi-quartz wedge plate 0*3° ; Calderon ocular, about 

 0*4° ; Bertrand ocular, about 0"5°. The sensitive tint plate is 

 useless because of strong natural color of mineral which veils 

 the true interference colors. 



The results of these tests show that the theoretical deduc- 

 tions from the general equations are in general valid, but that 

 in certain instances other factors, as natural color and very 

 low birefringence, become dominant and tend to render some 

 of the methods less sensitive and to favor the use of other, in 

 general less suitable methods. The bi-quartz wedge plate, 

 however, seems to apply in all cases with equally favorable 



