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



to pass through the upper uicols by simply revolving the same 

 and at every instant to state what percentage is passing through. 

 The particular angle of revolution for which the intensity of 

 light transmitted is best suited for the maximum sensitiveness 

 of the eye of the observer can be readily ascertained, and the 

 actual position of extinction for any given mineral plate be 

 determined by its use. By means of the bi-nicol ocular, the 

 adjustment of two crossed nicol prisms can also be tested accu- 

 rately and easily. 



This ocular, although serviceable, suffers from one defect 

 which it is difficult to overcome satisfactorily, namely, the 

 depolarizing of the total reflecting prism pairs on light waves 

 transmitted when the planes of the revolving nicols are not 

 parallel with the planes of the polarizer and- analyzer. As a 

 result a certain amount of false light is introduced into the 

 held and tends to veil the sharp contrast of the two halves and 

 thus to decrease the sensitiveness of the instrument. 



Bertrand plate — In place of birefracting plates, which 

 introduce an entirely new set of conditions in the path of light 

 waves and which complicate the expression for the relative 

 intensity correspondingly, Klein* and Bertrandf have used 

 the rotary power of quartz plates, cut normal to the principal 

 axis, on the plane of polarization of normally incident, plane 

 polarized waves. As shown above, the total eifect of such 

 a quartz plate in monochromatic light is merely to increase the 

 angular distance in the intensity formula (6). This power 

 of rotation of quartz varies with different wave lengths and 

 with the thickness of the plate. If white light be used, inter- 

 ference colors result. 



A quartz plate 7'50 mm thick shows the sensitive tint under 

 crossed nicols and can be used to good advantage in measuring 

 extinction angles, since for slight deviations of the crystal plate 

 from the position of true extinction its interference color rises 

 or falls. To increase its sensitiveness, Bertrand combiued two 

 plates (2 mm in thickness) of right-handed with two plates of 

 left-handed quartz, so that each right-handed plate is adjacent 

 to a left-handed plate. This plate is inserted in the focal plane 

 of the ocular and the sharp junction lines serve as crosshairs. 

 The Bertrand plate can be used in monochromatic light, pro- 

 vided for the particular wave length used, its angle of rotation 

 is not a multiple of 7r, in which case darkness ensues and the 

 observed effect is nil. By revolving the upper nicol it is pos-. 

 sible in white light to bring out the sensitive interference tint 

 over the entire held covered by the Bertrand plate, and in such 

 a position a very slight turn of an intervening crystal from its 

 position of true extinction is sufficient to disturb this equality 



* Neues Jalirbuch, 1874, p. 9. f Zeitschr. Kryst., i, 69, 1877. 



