86 Professor Airy on the 



And on tinning 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 in this and the last article apply equally well, sup- 

 posing the analyzing plate turned in the opposite direction. 



VII. If FresnePs 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. 12. If the 

 rhomb be placed in position 135°, 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 

 plate glass, but with the angles given by Fresnel for crown 

 glass) there is at the center an extremely dilute tint of pink : 

 I think it likely that this arises from the error in the angles, 

 as the intensity of the colour bears no proportion to that in 

 other parts of the spirals. The figure was drawn from the ap- 

 pearances given by a plate of quartz 0,26 inch thick. 



VIII. If two plates of quartz of equal thickness, but cut 

 one from a right-handed and the other from a left-handed 

 crystal, be attached together, and put between the polarizing 

 and analyzing plates, the left-handed slice nearest to the polar- 

 izing plate, the appearance presented is that of fig. 13. Four 

 spirals (proceeding from a black cross in the center; which is 

 inclined to the plane of reflexion) cut a series of circles at every 

 quadrant. The points of intersection are in the plane of re- 

 flexion, and perpendicular to it. This is the simplest way of 

 describing the form : but if we followed the colours which gra- 

 duate most gently, we should say that the form of each is 

 alternately a spiral and circular arc, quadrant after quadrant. 



