204 Professor Airy on the 



elliptical light supplies only the ordinary ray, as the colours one 

 quarter of an order within these points, and one quarter of an 

 order beyond them, are the same. 



The conclusion, that the inner rings will be circles, and the 

 outer ones two spirals, follows easily from this consideration. 

 The intromitted elliptical light has a smaller minor axis than the 

 ordinary ray (which is the only one that it resembles) in the 

 directions nearest to the axis of the crystal, and therefore it will 

 produce curves analogous to those produced by plane-polarized 

 light ; that is, analogous to circles. But it has a larger minor 

 axis than the ordinary ray, in the directions far from the axis of 

 the crystal, and therefore it will produce curves analogous to 

 those produced by circularly-polarized light; that is, analogous 

 to two spirals infolding each other. There is no difficulty in 

 making a more accurate investigation, on the principles described 

 in my former Paper: but I have not done it for a reason that 

 will shortly appear. 



I have not yet had the opportunity of making measures which 

 are sufficient to point out the law that connects the ellipticity of 

 the rays with the angle that they make with the axis. The fol- 

 lowing points however are made out. One of the rays certainly 

 is right-handed elliptical, and the other certainly left-handed 

 elliptical (or so nearly, that no difference is distinguishable). 

 The major axis of one is certainly perpendicular to the principal 

 plane of the crystal, and the major axis of the other is certainly 

 in that plane. 



In some trials of measuring the ellipticities of the rays, I 

 seem to have arrived at the following conclusion. The propor- 

 tion of the axes of the ordinary ray is more nearly one of 

 equality than the proportion of the axes of the extraordinary 



