174 .D/\ Brewster on the communication of double 
Proposition III. 
If a plate of jelly partially indurated , is kept in a state of com- 
pression or dilatation till the induration is completed , it will 
acquire permanently the structure of doubly refracting crystals . 
I experienced considerable difficulty in endeavouring to fix 
a plate of jelly in a state of permanent distension. The first 
process which was successful, consisted of taking a plate of 
isinglass, and allowing its two extremities to indurate, while 
the intermediate part was kept moist between two plates of 
glass. The isinglass was suspended by one of its indurated 
extremities, and dilated by a weight hanging from the other. 
In this distended state it exhibited very brilliant fringes, and 
it preserved the same property when it was completely har- 
dened. 
In order to obtain more perfect specimens, and a greater 
variety of forms I poured fluid isinglass into troughs of dif- 
ferent shapes made either of glass or of soft porous wood. 
The effect produced by transmitting polarised light perpendi- 
cularly through one of these troughs, is shown in Fig. si. 
(PL X.) ; and when the plate was inclined in the direction 
AB, it had the appearance shown in Fig, 22. (PL X.) After 
standing another day it exhibited, at a vertical incidence, the 
fringes shown in Fig. 23, (PL X.) where A mD, B mC, are 
the black spaces, and E, F, G, H, the tints of the first order 
of colours extending to the indigo of the second order ; but 
by inclining the plate in the direction AB, the dark spaces 
A m D, B m C approximated at the points m } n, till they met 
