834 



Prof. Carl Barus on the 



as the effective thickness increased. With obliquity de- 

 creasing through zero, moreover, the black bands between 

 crossed nicols moved in opposite directions when the normal 

 position was passed. For the polarizer at 45° both families 

 of ellipses were in the field at once. Either one or the other 

 set could be excluded by rotating the analyser. In the 

 absence of this, the field showed the ordinary or the extra- 

 ordinary set of ellipses separately, on rotating the polarizer 

 90°. In the further absence of the polarizer, one frequently 

 observes dislocated rings, the dislocation being in multiple 

 along equidistant vertical planes. In such loci the ordinary 

 and extraordinary rings, therefore, successively passed into 

 each other. In general a figure which is blurred without 

 the polarizer is resolved into two clean figures by the 

 aid of it, one being definitely displaced with reference to the 

 other. 



A large number of experiments was made, not only with 

 crystals cut normally or parallel to the axis, but with plates 

 of crown and of flint glass. In most cases, however, the 

 plates were inadequately plane parallel, and the absolute 

 value of the index of refraction was distorted in view of the 

 insertion of the slightly wedge-shaped crystal. Table II. is 

 a sufficient example of these results, as a whole. The data 

 were obtained with a plate of quartz, here adequately true, 

 the axis of rotation being vertical and made parallel to the 

 face of the crystal by the device of a perforated axle, which 

 need not be described here. Unfortunately the face of the 

 crystal was somewhat small, so that large angles of incidence 

 were inadmissible. In such a case the axis of rotation should 

 pass symmetrically between the faces, which was not quite 

 feasible in the case of the given apparatus. 



Table II. 



Plane parallel plate of quartz (" rouge "), lozenge-shaped, 

 cut normal to the axis. <? = *5996 cm. ; fi Q — 1*54423 ; 

 b = '421 x 10~ 10 . Vertical axis of rotation. 



i. 



r. 



105XAN . 



Observed 



105XAN 6 . 



Observed 

 he. 



1-54423 



105 X^. 



0° 



0° 



34059 



1-54423 



34059 







20 



12-80 



35414 



1-54426 



35439 



1-54466 



40 



30 



18-88 



37211 



1-54465 



37280 



1-54575 



110 



40 



24-60 



39825 



1-54422 



39925 



1-54575 



153 



50 



2973 



43781 



1-54827 



43944 



1-55063 



236 



