Professor Airy on the Double Refraction of Quartz. 329 



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

 glass) there is at the centre 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 0,36 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 crys- 

 tal, be attached together, and put between the polarizing and 

 analyzing plates, the left-handed slice nearest to the polarizino- 

 plate, the appearance presented is that of Fig. 11. Four spirals 

 (proceeding from a black cross in the centre, which is inclined 

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

 drant. The points of intersection are in the plane of reflexion, 

 and perpendicular to it. This is the simplest way of describing 

 the form : but if we followed the colours which graduate most 

 gently, we should say that the form of each is alternately a spi- 

 ral and circular arc, quadrant after quadrant. At a distance 

 from the centre the black brushes are seen. If the combination 

 be turned so that the right-handed slice is nearest to the pola- 

 rizing plate, the spirals are turned in the opposite direction. 

 This is one of the most beautiful phenomena of optics. The 

 slices from whose appearance the figure was drawn are each 0,16 

 inch thick. 



[Mr Airy next proceeds to explain the mode of calculating 

 these phenomena on assumed laws of the nature of light in the 

 two rays of crystals ; but as this investigation occupies thirty-five 

 pages, intelligible only to the mathematical reader, we arc ob- 

 liged to refer such readers to the original Memoir.] 



From the agreement between the observed and the calculated 

 appearances, I think there is little doubt that the nature of the 

 light in the two rays of quartz is such as I have described. I 

 do not mean to exclude the possibility of supposing that the 

 form of neither wave (in the construction for determining the 

 course of the rays) is exactly spherical or exactly spheroidal 

 provided the difference of the forms be nearly the same as that 

 of a sphere and a spheroid. Nor do I mean to assert that each 

 elliptically-polarizcd ray consists exactly of two plane polarized 

 r&yi following each other at the interval of one-fourth of ail un- 



