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XVI. On the Development and Extinction of regular doubly refracting Structures in the 
Crystalline Lenses of Animals after Death. By Sir David Brewster, K . J 1 . 
LL.D. F.R.S. 8fc. fyc. 
Received May 10, — Read 1st June, 1837. 
Since the year 1816, when I communicated to the Royal Society an account of the 
doubly refracting structures which exist in the crystalline lenses of fishes and other 
animals, I have examined a great variety of recent lenses, with the view of ascertain- 
ing the origin of these structures, the order of their succession in different lenses, 
and the purpose which they answered in the animal ceconomy. Although I had 
found that in the lenses of the cod, the salmon, the haddock, the frog-fish, the skate, 
and several other fishes there were three structures, the innermost of which had 
negative double refraction, the next positive , and the outermost negative double re- 
fraction, yet in the lenses of animals the greatest discrepancies presented themselves. 
In every case, however, excepting one, I have found the central structure in all qua- 
drupeds* to be positive, while it is always negative in fishes when there are three 
structures, but this positive structure sometimes existed alone, with faint traces of a 
negative structure ; sometimes it was followed by another positive structure, separated 
from the first by a black neutral circle, in which the double refraction disappeared. 
Sometimes these two positive structures were succeeded by an external negative struc- 
ture. Sometimes the central and external positive structures were separated by a 
negative structure, and at other times the lens exhibited four structures, a negative 
and a positive one alternating. As these discrepancies appeared in the lenses of 
animals of the same species, I conceived that they were owing to differences of age or 
sex, or to some change in the health of the animal. I was therefore led to make new 
observations in reference to these probabilities, and to observe the phenomena with 
additional attention when the structure differed from that which was most common. 
In these observations I sometimes noticed in the dark or neutral line, which separated 
two positive structures, something like a trace of an intervening structure, which was 
either about to disappear, or about to be developed. This conjecture was confirmed 
by observations on the lenses of a cow eleven years old. 
The lenses after being carefully taken out, were freed from the adhering portions 
of the vitreous humour by the gentle application of blotting paper, so as not to disturb 
their internal structure. The lenses were elliptical. Their longest diameter was 0*774 
inch, their shortest diameter 0*747 inch, and their thickness 0*513 of an inch. 
The first lens which I exposed to polarized light was in the highest perfection, and 
the symmetry of the optical figure unusually beautiful. I have represented it in 
* Excepting the hare. See Phil. Trans. 1836, p. 37. 
2 L 2 
