F. E. Wright — Measurement of Extinction Angles. 379 



shown in fig. 10 above, which was constructed in the workshop 

 of the Geophysical Laboratory. 



Methods involving revolution of upper nicol. — In all of the 

 preceding methods the nicols have been considered crossed and 

 the crystal plate revolved. The intensity formula shows, 

 however, that the relative intensity is dependent not only on 

 the angle 6 of the crystal plate but also on <j>, the angle between 

 the principal planes of the nicols. It was shown in the general 

 mathematical treatment that this method is in general at least 

 twice as sensitive as the method based on the revolution of the 

 crystal plate under crossed nicols. The mode of application 

 of this method to any particular crystal plate is obvious and 

 consists simply in placing the crystal under crossed nicols in 

 its position of apparent true extinction and then observing, 

 either in white or monochromatic light, the changes which 

 occur on revolving the upper or lower nicol through small 

 angles with its normal position. In case the crystal is actually 

 in its position of true extinction, the crystal and field attain 

 their position of maximum darkness simultaneously and show 

 the same increase in its intensity of illumination ; if, however, 

 the crystal be not in its position of true extinction, but a 

 small + angle, as 30' distant, then for a position of the nicol 

 + 2° from its normal position, the crystal plate will appear 

 lighter than the field ; and vice versa for the nicol —2° 

 from its normal position the crystal plate will appear darker 

 than the field. This method is extremely simple in manipu- 

 lation and does not require special apparatus, but seems not 

 to have been applied before to the measurement of extinction 

 angles. Weinschenk,* in describing the adjustment of the 

 nicols in the microscope, uses the interference phenomena 

 which occur under these conditions, but does not appear to 

 have applied conversely the principle to the practical deter- 

 mination of the optic ellipsoidal axis in a given crystal plate. 



To double the sensitiveness of this method of revolving the 

 upper nicol, the bi-nicol ocular attachment of fig. 11 can be 

 used. By this device alone, without the upper nicol used in 

 the above methods, the two halves of the field in the ocular 

 preserve the same intensity of illumination at every instant, 

 provided no disturbing crystal plate intervenes." If the posi- 

 tion of the latter does not coincide precisely with its true 

 position of zero extinction, the two halves of the field appear 

 unequally illuminated and by revolving the nicols that position 

 of the nicols can be found for which the effect is most pro- 

 nounced for a given angle 6. 



In its effect the bi-quartz wedge plate is identical with the 

 revolving bi-nicol scheme, and has the advantage of requiring 



* Zeitschr. Krystall. xxiv, 581-583, 1895. 



