26 MR. TALBOT ON THE OPTICAL PHENOMENA OF CERTAIN CRYSTALS. 
of view being dark, the little circles become luminous, and we see upon each of them 
a well-defined and dark cross, dividing the crystal into four equal parts. All these 
crosses are placed similarly, and are parallel to each other, and their direction remains 
unaltered when the crystals are turned round in their own plane by revolving the 
plate of glass upon which they stand. This beautiful appearance can be seen with a 
moderate magnifying power. I measured the diameter of some of the larger crystals, 
which I found to be from to -g— of an inch. But there are many much smaller, 
and indeed they may be seen decreasing in size, until nothing remains visible of their 
structure but the four luminous quadrants, appearing like four minute dots of co- 
loured light placed close together. 
I proceeded to examine the circles with a high magnifying power, and under fa- 
vourable circumstances of illumination, and I observed in them a very admirable 
structure. 
Each circle has upon it one or more coloured rings arranged concentrically, but 
the number as well as the colour of these rings is different in different individuals. 
The innermost ring is deeply coloured or black, and incloses a central space of 
white light, which is traversed by the arms of the cross intersecting in the centre. 
This part of the cross, which stands within the innermost ring, is beautifully well de- 
defined, and perfectly black. The general appearance resembles the figure 98, in 
Brewster’s Optics, which is a representation of the rings seen in uniaxal crystals. It 
especially resembles it in the circumstance above mentioned, viz. the more defined 
outline of the part of the cross which is within the innermost ring. 
We have hitherto supposed the polarizers to be crossed, but if we place them in a 
parallel position we shall see a phenomenon complementary to the above. The circle 
now presents four patches of coloured light, one in each quadrant ; and we generally 
see near the centre four black or obscure spots, which correspond to the arms of the 
cross in the other position. 
Such is an outline of the microscopic appearances presented by these little crystals, 
which are probably the minutest bodies in which so complicated an optical structure 
has hitherto been witnessed. I find that the smaller the circles are, the more perfect 
is their form and the brighter their colours. 
These crystals, as I have already observed, probably consist of spicula diverging 
from a point, but which are in the closest possible contact, and in a state of complete 
mechanical cohesion. It seems to follow as a consequence from such a structure that 
their density must increase from their circumference towards their centre. Nowit is 
worthy of remark, that Sir David Brewster has discovered very similar phenomena 
by polarized light in the crystalline lenses of certain fishes, which are known by direct 
experiment to increase in density towards the centre. Indeed the figure which he has 
given of the lens of the codfish in the Philosophical Transactions for 1816 (Plate XII. 
fig. 1.) is so like the appearance of one of the crystals which I have described, that it 
might be supposed to have been intended for a representation of it. 
