Dr. Brewster on the effects of simple pressure, &c. 6 1 
produced such a change, as to communicate to it the property 
of perpendicular depolarisation. 
In whatever manner this difference of action was produced, 
the effects of pressure seemed to require farther investigation, 
and in order to be able to apply a sufficient force, without 
injuring the structure of the substance, I employed animal 
jellies which could be brought to any degree of tenacity with- 
out losing their transparency. 
Having cut out of newly made calves' feet jelly, a cylin- 
drical portion, about half an inch broad and half an inch high, 
I placed it between two plates of glass, and observed that it 
did not possess, in the slightest degree, the prope^r of depo- 
larising light. After standing some days, it began to depo- 
larise light at its circumference, and in the course of fifteen 
days this property gradually extended to its central parts. 
The cylinder of jelly had at first such a slight degree of te- 
nacity, that it quivered with the gentlest motion ; it was now 
however considerably indurated, and though it formed a plate 
exactly parallel, yet it diverged the incident rays like a con- 
cave lens, from the external parts having a greater degree 
of induration, and consequently a higher refractive power 
than the parts towards the centre. At the end of three weeks 
it began to lose its transparency, and at the same time its 
depolarising structure ; and in the course of a few days more, 
it had no more action upon light than a mass of water. Its 
thickness was now reduced, by contraction, to about one- 
seventh of an inch, and it possessed a degree of tenacity, ap- 
proaching to that of caoutchouc, which enabled it to sustain, 
without injury, a very considerable degree of pressure. 
In this state, I exposed the plate of jelly to the light of a 
