Structure of doubly refracting crystals. 397 



By comparing, in a rude manner, the coloured rings formed 

 by different bodies, with the thickness of the plates by which 

 they were produced, I concluded that the conjugate diameters 



of the rings were nearly as -, — -.^ 3 , m being the index of re--' 



fraction. In the nitrate of potash, however, their magnitude is 

 quite anomalous, as it produces along the axis of the hexaedral 

 prism a series of miniature rings, nearly eight times less than 

 they should have been according to the preceding law. The 

 beautiful generalisation of the phenomena of coloured rings, 

 which we owe to the genius of the celebrated Biot, may per- 

 haps afford an explanation of this apparent anomaly. 



The Carbonate of Potash forms also two images, one of which 

 is bright, and the other nebulous. They are polarised in an 

 opposite manner, like those formed by the nitrate of potash, ^ 

 but the nebulous image is more distinct in the carbonate. 

 With a prism bounded by natural faces, and having a refrac-'* 

 ting angle of 49" 53', I obtained the following measures of its 

 mean refractive power : ' 



Index of refraction for the nebulous image, 1.379 

 Index of refraction for the bright image, 1.482. 



IV. On the Structure of. Doubly Refracting Crystals. 



Notwithstanding the numerous discoveries which have re-, 

 cently appeared respecting, the polarisation of light, no at-~ 

 tempt has been made to apply them in solving the pro- 

 blem of double refraction. Theyfurnish us, indeed, with a va- 

 riety of beautiful phenomena, analogous to the polarisation of 

 light, which always accompanies the production of two images ; 

 but they afford no ground of conjecture respecting the separa- 

 tion of the penci into two parts. 



When. 



