Double Refraction of Quartz. 85 



interposed between the crossed plates, a set of rings is seen as in 

 figs 7, 8, 9, 10. As far as the eye can judge, the rings are ex- 

 actly circular, but there is no black cross, and the central tint 

 is not black, but removed from it by a number of tints in 

 Newton's scale proportional to the thickness of the quartz. Thus 

 with a thickness 0,48 inch, the central tint is pale pink : with 

 a thickness 0,38 inch, the central tint is bright yellowish green: 

 with thickness 0,26 inch, it is a rich red plum-colour: with 

 thickness 0,17 inch : it is a rich yellow. 



The colours then appear to be nearly the same, beginning 

 from the center, as in Newton's scale, beginning with the tint 

 representing this central tint. At a considerable distance from 

 the center four dark brushes begin to be visible, in the same 

 directions as the arms of the black cross in calc spar. 



V. Now (supposing the crystal right-handed), if the plate 

 of quartz be thin, and the analyzing plate be turned, the upper 

 part towards the observer's left hand, a blueish short-armed 

 cross appears in the center*, which on turning further becomes 

 yellow: and the rings are enlarged. On turning still further, 

 the cross breaks into four dots. The rings are no longer cir- 

 cular, but of a form intermediate between a circle and a square, 

 their diagonals (as well as the cross) being inclined to the left 

 of the parallel and perpendicular to the plane of reflexion. See 

 fig. 11. If the analyzing plate be turned the other way, there 

 is no cross: the form of the rings is changed from circular 

 nearly as in the former case. 



VI. If the plate of quartz be thick, the dilatation of tht- 

 nngs and the change of form are all the perceptible phenomena. 



* This may be considered as the definition of right-handedness of the crystal : and this 

 observation gives the readiest means, with a thin plate, of determining whether it is right- 

 handed or left-handed. If the plate be thick, the easiest method is to observe in which 

 direction the analyzing plate must be turned to make the rings dilate. 



