Addition to a Paper "On the Nature of the Light in 

 the Two Rays produced by the Double Refraction 

 of Quartz" 



By G. B. AIRY, M.A.; M.G.S.; 



LATE FELLOW OF TRINITY COLLEGE; PLUMIAN PROFESSOR OF ASTRONOMY AND 



EXPERIMENTAL PHILOSOPHY IN THE UNIVERSITY OF CAMBRIDGE; AND FELLOW OF THE 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 



[Read April 18, 1831.] 



After I had made the experiments described in a paper which 

 was read to this Society on February 11, 1831, I received from 

 Mr. Dollond an apparatus constructed under my direction, which 

 for convenience and extent of application exceeds any other that 

 I have seen. The parallel rays that fall on a piece of plate glass 

 blackened at the back are reflected, all completely polarized, and 

 are received on the first lens, which makes them all pass through 

 one point: then diverging- they are received on the second lens, 

 whose distance from that point is equal to its focal length, and 

 they emerge from it parallel. In this state they are received on 

 the analyzing plate (a piece of plate glass blackened behind) and 

 all are of course completely analyzed. They are then received 

 on the third lens (fixed in a sliding- tube, like the eyeglass of a 

 telescope) which makes them all pass through the eyehole. The 

 lenses are of equal focal length, and their arrangement is pre- 

 cisely the same as that of the lenses in the old three-glass eye- 



