126 On the Structure 



rings were nearly as t -, m being the index of refraction. In 



the nitrate of potash, however, their magnitude is quite anoma- 

 lous, as it produces along the axis of the hexaedral prism a serie'; 

 of miniature rings, nearly ei^ht times less than they should have 

 been according to the preceding law. The beautiful generaliza- 

 tion of the phaenomena of coloured rings, which wc owe to the 

 genius of the celebrated Biot, may perhaps afford an explanation 

 of this apparent anomaly. 



The carbonate of potash forms also two image.'?, one of which 

 is bright, and the other nebulous. They are polarized 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 refracting angle of 

 49° 53', I obtained the following measures of its mean refractive 

 power : 



Indfex of refraction for the nebulous image . 1'37D 

 Index of refraction for the bright image . . r482 



IV. On the Struchire of douhly refracting Crystals. 



Notwithstanding the numerous discoveries which have recently 

 appeared respecting the polarization of light, no attempt has 

 been made to apply them in solving the problem of double re- 

 fraction. They furnish us, indeed, with a variety of beautiful 

 phaenomena, analogous to the polarization of light, which always 

 accompanies the production of two images ; but they afford no 

 ground of coijecture respecting the separation of the pencil into 

 two parts. 



When I discovered the property possessed by tlie agate, of 

 foruiing a bright and a nebulous image, and of polarizing them in 

 an opposite manner, like all doulily refracting crystals, I M^as suffi- 

 ciently aware of the conclusions which it authorized*; but as 

 no other crystallized body exhibited analogous phsenomena, I 

 contented myself with stating these conclusions a.s mere conjec- 

 tures, which required the sanction of numerous experiments. 



In the carbonate of barytes, however, and in tbe nitrate and 

 carbonate of potash, we are presented with jiroperties analogous 

 to those of the agate, and are therefore enabled to resume this 

 subject, with that confidence wliich can only be derived from 

 multiplied observations. 



When we examine the two images formed by calcareous spar 

 and other perfectly transparent crystals, we find that they have 

 the same magnitude, and are equally luminous and distinct. 

 There is, therefore, no circumstance which can lead us to sup- 

 pose, that the light which forms the one image passes through a 



* See Phil. Trans. Lond. 1813, part i p 101. 



part 



