200 Professor Airy on the 



piece, measuring the distance between the second and third lenses 

 along the incident and reflected ray. The analyzing plate, with 

 the second and third lenses, turns on a spindle parallel to the 

 rays polarized by the first plate, whose direction passes through 

 the centers of the first and second lenses. To see the rings &c. 

 produced by a crystal, it should be placed at the point where the 

 rays cross between the first and second lens: and a specimen 

 ^ inch broad so placed will shew the rings with perfect brilliancy 

 and clearness in their utmost extent. If it is wished to use a 

 micrometer, the micrometer should be placed between the polari- 

 zing plate and the first lens, at a distance from the latter equal 

 to its focal length, where it will be seen distinctly at the same 

 time that the rings are seen distinctly. If it is wished to see the 

 macled structure of quartz, amethyst, topaz, &c the crystal is to 

 be placed in the situation assigned for the micrometer : then no 

 rings will be seen, but on turning the analyzing apparatus round 

 its spindle the different parts will be differently coloured. For 

 the use of plane-polarized light only, there is no need for a po- 

 larizing plate larger than the projection of the first lens: and the 

 distance between the nearest edge of the polarizing plate and the 

 first lens needs not to exceed by a large quantity the focal length 

 of the latter. But as I now consider every apparatus incomplete 

 which does not allow of the use of circularly and elliptically 

 polarized light, I have had the polarizing plate separated so far 

 from the lens that it allows Fresnel's rhomb (mounted as in 

 Plate 8, fig. 4) to be placed between them, room being left for 

 the micrometer &c. between the rhomb and the lens: and the 

 polarizing plate is made so large that it will transmit plane- 

 polarized light to the end of the rhomb, at whatever angle it is 

 placed, while the other end is centrally opposite to the first lens. 



