XVI. On the development and extinction of regular doubly refracting structures in the crystalline lenses of animals after death

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Abstract
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 ascertain­ing the origin of these structures, the order of their succession in different lenses, and the purpose which they answered in the animal œconomy. 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 re­fraction, 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 quadrupeds 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 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 struc­ture. 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 Plate XV. fig. 1., in which only two structures, or two series of positive sectors, are visible. The lens was now a day old, and there seemed to be a faint light within the two black rings, especially in the outer one, which was either the remains of an old, or the germ of a new structure. If this were the case, then the anomalous combination of two positive structures would be converted into a combination of four structures, in which a negative and a positive one alternated.

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