! 7 2 Dr. Brewster on the communication of double 
SECT. II. On the communication of double refraction either 
transiently or permanently to animal jellies by gradual indu- 
ration > and by mechanical compression and dilatation. 
Proposition L 
When a plate of animal jelly 3 either approaching to fluidity , or in 
a state of high elasticity is compressed or dilated , it possesses 
the same optical properties as compressed or dilated glass. 
It would be unnecessary labour to detail the numerous expe- 
riments by which I obtained from animal jellies, the various 
results described in the preceding Section.’* I shall, therefore, 
content myself with pointing out a very simple method by % 
which the experiments may be easily repeated. Let a paral- 
lelopiped of isinglass EF, Fig. 20, (PI. X.) newly coagulated, 
be cemented by isinglass of the same consistency to two plates 
of glass AB, CD. By forcing the plates together, so as to 
compress the jelly, various orders’ of colours will be developed 
at mn, having the same character as the external fringes of 
crystallized glass. When the pressure is removed, two black 
fringes meet, as it were, at m n, and upon separating the 
plates, so as to dilate the jelly, another set of fringes will 
appear at m n , having a character opposite to that of the other 
fringes. If we force the plates together obliquely, so as to 
form an angle, and thus compress the jelly on one side, and 
dilate it on the other, the two opposite sets of fringes will be 
distinctly seen. 
When the plates are pressed together with such force as to 
* See Phil. Trans. 1814, p. 60. where I have given an account of the discovery of 
this property of animal jellies. 
