F. E. Wright — Measurement of Extinction Angles. 383 



the crystal plate for a specified angular distance from its posi- 

 tion of trite extinction during the revolution of the upper nicol 

 from SS C to 9:2 C . These curves indicate that the probable 

 error with this method is less than half as great as in the 

 preceding method, for if the crystal be only ±15 distant from 

 its position oi true extinction, differences in intensity can even 

 then ited on revolving the upper nicol. 



The changes in intensity of illumination of the microscopic- 

 field on revolution of the analyzer are indicated by the 0' 

 curve, while for the crystal plate the 15' curve is applicable. 

 A: ^ : 48 fig. 3) the field is jost beginning to show deteet- 

 illumination Os per cent of total intensity), while for 

 the same angle the crystal is illuminated with 0*97 per cent 

 : the total intensity, nearly twice as great and easily notice- 

 able. In this position the crystal plate appears, therefore, 

 decidedly lighter than the field. On the other side of 90° the 

 crystal plate passes its limit of light sensibility under the 

 assumed conditions at 91° 42'. while for the same angle the 

 microscopic field is illuminated by -97 per cent of the total 

 intensity : in this case the field is appreciably brighter than 

 the crystal and the difference can be readily detected by the 

 eye. 



If white light be used, these differences are accentuated by 

 the abnormal interference colors which appear in the crystal 

 plate when it is not precisely in the p< siti f true extinction. 

 This method of revolving the upper nicol has the advantage, 

 furthermore, of not being dependent on the accuracy with 

 which the nicols are crossed, since all data are referred at once 

 to the plane of the analyzer. It is not. however, so advanta- 

 geous in very weakly birefracting or deeply colored mineral 

 plate-. 



The sensitiveness of the latter method can. moreover, be 

 doubled by devices which allow the phenomena on both sides 

 of tn obs srved simultaneously. This is the 



end striven for in the ocular plate, the Bravais-Stober plate, 

 the Calderon plate, the Tranbe plate, and accomplished most 

 effectively by the new circularly polarizing bi-qnartz wedge 

 and also by the bi-nicol ocular, though less satisfactorily. In 

 eae:. se last two devices the plane of polarization of 



the incident waves is turned through equal angles on both 

 - - ::' the' junction line of the two parts. so that the field 

 appears equally lighted throughout, while if the crystal plate 

 be not in its position of true extinction, it will appear lio-hter 

 than the field in the one half and darker in the second, Since. 

 however, there is an angle best suited under the given condi- 

 tions to show the-e differences most clearly, it follows that the 

 rt-ults can be had with a plate or apparatus in which the 



