in producing depolarisation. 63 
extremely soft that it could scarcely support itself in a vertical 
position. Having exposed it to a polarised ray, as before, I 
pressed its cylindrical circumference between my finger and 
thumb, and thus enabled it to restore the vanished image. A 
slight pressure, indeed, from one finger, was capable of pro- 
ducing a perceptible degree of depolarisation. 
Instead of calves' feet jelly, I next employed isinglass , 
brought nearly to the consistency of caoutchouc. After 
standing a day, the isinglass had, of its own accord, acquired 
the depolarising structure, even when cut into very thin films, 
either parallel or perpendicular to the surface; but upon 
placing a cake of it, about a quarter of an inch thick, between 
two plates of glass, and exposing it to polarised light, I found 
that the complementary colours were developed in a most 
beautiful manner by hard pressure, and I often saw a portion 
of a red and a blue ring upon one of the images of the candle, 
while the colours complementary to these occupied the other 
image. By varying the pressure new colours arose, and 
when the pressure was removed, the complementary tints 
gradually disappeared. As these changes of colour might be 
ascribed to the pressure, only in so far as it reduced the cake 
of isinglass to the degree of thickness necessary for their 
production, I brought the cake to the same thickness which 
it possessed when exposed to the pressure that developed the 
most lively colours. No colours, however, were now visible, 
but they were instantly reproduced, as before, by the appli- 
cation of pressure. 
I now melted the isinglass between two plates of glass, and 
allowed it to stand till it coagulated, which took place in less 
than a quarter of an hour. Upon transmitting through it a 
