F. E. Wright — Measurement of Extinction Angles. 377 



of interference color and to divide the field into four quad- 

 rants, the opposite sections of which are similarly colored, 

 while adjacent sections are differently colored. 



The Bertrand plate is best adapted for use in white light, 

 although it is possible to use it in monochromatic light pro- 

 vided its thickness be correct for the particular wave length 

 employed. 



The quartz half shade plate of S. Nakamura. — In a recent 

 paper,* S. Xakamura discussed the problem of the sensitive- 

 ness of the half shade system and arrived at practically the 

 same conclusions as those noted above. He suggests the 

 use of a double quartz plate of '± mm thickness instead of 

 S'o mm or 7 mm thick as in the Bertrand ocular, and by actual tests 

 finds the theoretical deductions valid and the plate useful. The 

 thickness of '4: mm is equivalent to an angle (90 — 9) of about 

 S*67° on each side of the junction line ; under certain con- 

 ditions of illumination this angle is undoubtedly the best, and 

 with the plate the accuracy of the measurements thereby 

 attained equal to that of any of the other measuring devices. 



Bi-quartz wedge-plate. — It is possible, however, to construct 

 a combination wedge of quartz plates of such a character that 

 any angle of rotation from 0° to any other value, positive or 

 negative, can be had on insertion of the wedge, thus adapting 

 to wedge form the advantage of the revolving bi-nicol ocular. 

 This has been accomplished by combining two plates of 

 quartz cut normal to an axis and of specified thickness, the 

 one of right-handed, the other of left-handed quartz, each 

 with a wedge of quartz of the opposite sign of rotary polari- 

 zation, as indicated in figure 12. f 



The effect of this combination is to produce zero rotation in 

 each half wedge where plate and wedge have the same thick- 

 ness and as the wedge is inserted or drawn out from this point 

 of zero rotation the angle of rotation increases proportionately 

 and in a positive sense on one side of the junction line of the 

 combination and in a negative seuse on the opposite half. This 

 combination wedge, which is introduced at the focal plane of 

 the ocular, divides the field under crossed nicolsinto two halves, 

 the intensity of color of which at any instant is equal, provided 

 no intervening crystal plate is present or is rendered inactive 



* Centralblatt f. Mineral ogie. 1905. 267-279. Compare also J. Mace de 

 Lepinay. Jour, de Phys. (2), iv. 267. 1885; (3), ix, 585, 19U0. Unfortunately this 

 paper did not come to notice until after the manuscript of the present article 

 had been sent to the press and the mathematical discussion by S. Nakamura, 

 which considers the problem from a somewhat different standpoint, could 

 not well receive the analysis and recognition which otherwise might have 

 been given it in the general theoretical part. 



I Made for the writer by Steeg & Reuter of Homburg v. d. Hohe, Ger- 

 many. Cost, 100 marks. The accuracy of this wedge was tested by the 

 writer and the grinding found to be exceptionally perfect. 



