62 Dr. Brewster on the effects of simple pressure 
candle polarised by reflection , and employing a prism of Iceland 
spar, one of the images of the candle vanished at every qua- 
drant of its circular motion, just as if the jelly had not been 
interposed. I now pressed together the two plates of glass, 
that inclosed the cake of jelly, and was surprised to find that 
the vanished image was restored, the light being depolarised 
in every position of the cake. Upon removing the pressure, 
the image again vanished, and the cake resumed its uncrys- 
tallized state. 
In order to vary the experiment, I took two prisms of Ice- 
land spar, and having put them in such a position that two of 
the four images of a candle vanished ; I then placed the cake 
of jelly between the prisms, and pressing them hard together, 
the two vanished images were restored, the depolarisation 
being more perfect as the pressure was increased. The removal 
of the pressure again destroyed the depolarising structure. 
I repeated the preceding experiments with another plate of 
jelly, which was perfectly transparent, and which possessed 
the depolarising structure only at the edges. The pressure 
never failed to communicate to the central parts the property 
of depolarisation, and I repeatedly observed that the comple- 
mentary colours produced by topaz, &c.* were imperfectly 
exhibited when the pressure had attained a particular mag- 
nitude. 
As the cakes of jelly used in the preceding experiments, 
had both been crystallized by induration, I took another cylin- 
drical portion that had never possessed that crystalline struc- 
ture which is necessary to depolarise light. It was about an 
inch high, and three quarters of an inch broad, and was so 
* See Phil. Trans. 1814. Part. I. 
