202 Professor Airy on the 



On considering' my hypotheses relative to the nature of the 

 two rays of quartz, the following method suggested itself as a 

 means of verifying one part of the hypotheses, and as affording 

 a power of measuring the ellipticity of the rays. Suppose (by 

 placing Fresnel's rhomb in a position between 0° and 45°, or 

 between 90° and 135°) elliptically polarized light is made to pass 

 through quartz. Whether this be right-handed or left-handed, 

 there is one direction (A) in which one ray of the quartz (sup- 

 pose for instance the ordinary) is of just the same kind as the 

 incident elliptical light. Consequently that light furnishes no 

 extraordinary ray. Now if we take a direction (a) nearer to the 

 axis by the smallest possible angle, and another (b) further from 

 the axis by the smallest possible angle, than the direction just 

 mentioned (A), the same elliptical light incident in the directions 

 (a) and (b) will furnish extraordinary rays, but the paths of these 

 extraordinary rays will differ (independently of all other causes) by 

 half the length of a wave. For the elliptical light which is of 

 the same kind as the ordinary ray in (A) is more elliptical than 

 the ordinary ray in (a) and less so than that in {b). And there- 

 fore when we separate the elliptical light into an ordinary and 

 an extraordinary ray in (a), it is the defect of its minor axis 

 which produces the extraordinary ray: when we do the same 

 for (b), it is the excess of its minor axis which produces the 

 extraordinary ray. The vibration therefore which produces the 

 extraordinary ray in (a) being in the positive direction, that which 

 produces the extraordinary ray in (b) will be in the negative 

 direction, or vice versa. And this amounts to the same as re- 

 tardation or acceleration by half the length of a wave. It will 

 readily be seen that this is independent of the crystalline separa- 

 tion of the two rays, and is true however small be the angle 



