328 Professor Airy on the Double Refraction of Quartz. 



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 bluish short-armed cross 

 appears in the centre, (see Note F,) 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 

 circular, 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. 9. 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 the rings 

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

 on turning the analyzing plate continually to the left, the rings 

 continually dilate, and new spots start up continually in the 

 centre, and become rings. If the crystal be left-handed, the 

 remarks on this and the last article apply equally well, suppos- 

 ing the analyzing plate turned in the opposite direction. 



VII. If Fresnel's rhomb be placed in the position 45°, and 

 the light thus circularly polarized pass through the quartz ; on 

 applying the analyzing plate, instead of rings there are seen two 

 spirals mutually inwrapping each other as in Fig. 10. If the 

 rhomb be placed in position 1 35°, the figure is turned through 

 a quadrant. If the quartz be left-handed, the spirals are turned 

 in the opposite direction. The central tint appears to be white. 

 With the rhomb which I have commonly used (which is of 



