296 On Double Refraction. 
Arr. VII.—On the production of regular double refraction 1 the 
molecules of bodies by simple pressure, with observations on the 
origin of the doubly refracting structure ; by Davin Brewster, 
LL.D. F.R.S. L. & E.* 
[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 mechanic 
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 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 surla- 
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 1s 
pressed between two plates of glass by the heat of the hand, te 
film is almost perfectly transparent by transmitted light, though of @ 
milky white appearance by reflected light. It has not the prope 
of depolarization when the light is incident vertically ; but it posst® 
es it in a very perfect manner at an oblique incidence, and exhibits 
the segments of colored rings.” { 
* 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. 
t Ibid. 1815, pp. 31, 32. 
