92 



becomes tubular, afterwards vascular, and then takes the form of 

 healthy granulations. 



On the Laws of Polarization and Double Refraction in regularly Crystal- 

 lized Bodies. By David Brewster, LL.D. F.R.S. Land, and Edin. 

 InaLetter to theRightHon. Sir Joseph Banks, Bart. G.C.B. P.R.S. 

 Read January 15, 1818. [Phil. Trans. 1818, p. 199.] 



In the different inquiries which the author has already laid before 

 this Society, his attention was often directed to the phenomena of 

 regular crystals ; but he only lately succeeded in reducing under a 

 general principle all those complex appearances which result from 

 the combined action of more than one axis of double refraction. In 

 this paper Dr. Brewster gives a general view of the present state of 

 our knowledge respecting the double refraction and polarization of 

 light, and afterwards traces the steps which led him to the discovery 

 of the general law. He began his researches by the examination of 

 165 crystals, in 145 of which he discovered the property of double 

 refraction. In 80 he was able to ascertain whether they had one or 

 more axes ; and by examining the tints which they exhibited at va- 

 rious angular distances from the axes, whence the forces emanate, he 

 has been led to a general principle, which embraces all the phenomena 

 and extends to the most complex as well as to the most simple de- 

 velopment of the polarizing forces. This general principle, says Dr. 

 Brewster, is in no respect an empyrical expression of the facts which 

 it represents, nor is it supported by any empyrical data. Founded 

 on the principles of mechanics, it is a law rigorously physical, by 

 which we are enabled to calculate all the tints of the coloured rings, 

 and all the phenomena of double refraction, with as much accuracy 

 as we can compute the motions of the heavenly bodies. 



The faculty of depolarization, explained by the author in a former 

 paper, has been considered as sufficient indication of two separate 

 images ; and upon this principle it has been stated that all crystals 

 are doubly refractive whose primitive form is neither the cube nor 

 the regular octohedron : but this is incorrect ; for some of these 

 crystals possess a doubly refracting structure in a high degree. Ad- 

 mitting the statement, however, it could not have been used as a 

 rule for determining whether a crystal refracts doubly or singly ; for 

 it is more difficult to detect the primitive form than to examine the 

 optical properties. Tungstate of lime, for instance, would have been 

 reckoned a crystal without double refraction, when Haiiy believed its 

 primitive form to be the cube, although it is highly doubly refractive. 



In examining the nature and properties of the coloured rings pro- 

 duced by certain crystals, the author found that the squares of the 

 diameters of the rings were, in every case, proportional to the num- 

 bers which represent the corresponding tints in Newton's table. 



When a plate of beryl was combined with a plate of calcareous 

 spar, the system of rings was the same as would have been produced 

 by two plates of beryl, one of which was the plate employed, and 



