296 On Double Refraction. 



Art. Vll.-^On the production of regular double refraction in the 

 molecules of bodies by simple pressure, with observations on the 

 origin of the doubly refracting structure ; by David Brewster, 

 LL.D. F.R.S. L.&iE.* 



[Read before the Royal Society, February 11, 1830.] 



In various papers already printed in the Philosophical Transactions, 

 I have had occasion to show that the phenomena of double refraction 

 may be produced artificially, by certain changes in the mechanical 

 condition of hard and soft solids. f In all these cases the phenomena 

 are related to the form of the mass in which the change is induced ; 

 and in the case of hard and elastic solids, they vary with any varia- 

 tion of form which alters the mechanical state of the particles. In 

 isinglass and other bodies to which double refraction has been com- 

 municated by induration, the particles take a permanent position, 

 which is not altered by any change of shape ; but still the phenome- 

 na exhibited by a given portion of the mass are related to the surfa- 

 ces where the indurating cause operated, and also to those by which 

 the isinglass was bounded ; and they depend on the position which 

 that portion occupies in the general mass. 



In all these cases the phenomena are entirely different from those 

 of regular crystals, and in none of them is the doubly refracting 

 force a function of the angle which the incident ray forms with one 

 or more axes given in position. 



As long ago as 1814, I communicated to the Royal Society the 

 following experiment on the depolarizing structure of white wax and 

 resin : 



"When resin is mixed with an equal part of white wax, and is 

 pressed between two plates of glass by the heat of the hand, the 

 film is almost perfectly transparent by transmitted light, though of a 

 milky white appearance by reflected light. It has not the property 

 of depolarization when the light is incident vertically ; but it possess- 

 es it in a very perfect manner at an oblique incidence, and exhibits 

 the segments of colored rings."J 



* Received, by the kindness of the author, with several other of his more recent 

 papers on Light, which will appear in successive numbers of this Journal, 

 t Phil. Trans. 1814 ; 1815, pp. 1, 30, 60 ; 1816, pp. 46, 56. 

 } Ibid. 1815, pp. 31, 32. 



