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V. 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. 8$ E. 
Read 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 arti- 
ficially by certain changes in the mechanical condition of hard and soft solids * 
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 variation of form which alters the mechanical state of the particles. 
In isinglass and other bodies to which double refraction has been communi- 
cated by induration, the particles take a permanent position, which is not altered 
by any change of shape ; but still the phenomena exhibited by a given portion 
of the mass are related to the surfaces 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 : 
ee When resin is mixed with an equal part of white wax, and is pressed be- 
tween 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 re- 
flected light. It has not the property of depolarization when the light is inci- 
dent vertically ; but it possesses it in a very perfect manner at an oblique inci- 
dence, and exhibits the segments of coloured rings-i~”. 
* Phil. Trans. 1814; 1815, pp. 1, 30, 60; 1816, pp. 46, 56. 
f Ibid. 1815, pp. 31, 32. 
