330UBLY REFRACTING CRYSTALS. 301 



that the imperfect structure, which, in general, transmits only 

 a mass of nebulous light, allows a distinct image to be formed, 

 when the rays are incident in one particular direction ; while 

 the perfect structure, which in general gives a distinct image, 

 allows an imperfect image to be formed, when the light pene- 

 trates it by a particular path. 



These inferences, which I conceive to be irresistible, have a 

 higher degree of importance than we may at first be disposed 

 to attach to them. They form a real step in the explanation 

 of double images, and indicate a part of that structure which 

 is necessary to their formation. The other phenomena of 

 double refraction are still involved in obscurity, 'i he opposite 

 polarisation of the two pencils, may be explained by suppo- 

 sing the crystal to consist of laminae inclined in various direc- 

 tions ; and, as I have shewn in another place *, the same phe- 

 nomena may be actually produced by an artificial crystal, com- 

 posed of bundles of glass plates. The most perplexing point, 

 however, is the extraordinary refraction which takes place at a 

 perpendicular incidence. Whether this phenomenon is the re- 

 sult of an extraordinary law of refraction, as Huygens and 

 Newton supposed, or is produced by forces dependent on the 

 elementary structure of the crystal, is a question which still 

 remains to be determined. The extraordinary reflection and 

 refraction arising from the last of these causes, which I have 

 discovered in Mother of Pearl f, present an analogy, by no 

 means remote, to the phenomena of double refraction. 



* Phil. Trans. Lond. 1814, Part I. p. 230. 



•J- This substance, whose remarkable optical properties I have explained in 



another place, resembles the Agate, the Carbonate of Barytes, and the Nitrate 



and Carbonate of Potash, in giving a bright and a nebulous image, when the 



Vol. VII. P. II. Q q light 



