36 
SIR DAVID BREWSTER ON THE ANATOMICAL AND OPTICAL 
of the axis of the lens. When such an ambiguity, however, presents itself, we must 
observe carefully if the diffusion is more elongated in one direction than another. If 
it is not, and especially if there is seen a small circular dimple or depression at the 
poles when we follow the light reflected from the surface of the lens, there can be no 
doubt that the fibres converge to two opposite poles, like the meridians of a globe. 
But if the diffused polarity be of an oval form, and if the greater axis of the oval on 
one side of the lens be perpendicular to the same axis on the other side, we may then 
safely infer that the fibres are related, as in the lens of the salmon, to two septa at 
each extremity of the axis of vision. When these methods fail, the ambiguity may 
sometimes be removed by boiling the lenses, and observing the manner in which 
they crack at the poles. 
From a too hasty generalization of a small number of facts, Leeuwenhoek main- 
tained that all fishes and birds have two short septa ; and M. Sattig* has advanced the 
same opinion in reference to fishes, and committed the additional mistake of making 
every part of the fibres lie in the same plane. Dr. Thomas Young maintains that 
there are no septa in the lenses either of birds or fishes -f ; and in pursuing the chi- 
maera of a muscular lens, he not only renounced discoveries of his own, after Leeu- 
wenhoek, but adopted grave errors which have no foundation whatever either in ob- 
servation or analogy. 
It is a very remarkable circumstance, that the hare and the rabbit should be the 
only quadrupeds whose lenses have two septa, like that of the salmon. This fact was 
observed by Leeuwenhoek^ and also by Sattig, who remarks that the septa are 
larger in the lenses of these animals than in those of fishes. 
The fibres of the lenses of the hare and the rabbit are curves of contrary flexure, 
like those of the salmon, and they are distinctly toothed, though the teeth are much 
smaller than those of the salmon and the cod, of which I have given a representation 
in a former paper. 
Although I have stated that the hare and the rabbit are the only quadrupeds whose 
lenses are known to have the same structure as that of the salmon, yet there is a rare 
marsupial quadruped, the Perameles nasuta of Geoffroy, whose lens will probably be 
found to have two septa on each of its surfaces. Professor Grant was so good as to 
send me a single lens of this curious animal ; but as one of its faces was much in- 
jured, I was able only to discover the two septa on the side of the lens which was 
uninjured. It is highly probable that the fibres will have a similar arrangement on 
the other side of the lens; but until this is actually determined, it is possible that 
* Lentis Crystalline Structura Fibrosa. Preside Iteil Defendit Samuel Godofredius Sattig Silesius. 
Hale, 1794. It is not surprising that the author of this thesis should have followed Leeuwenhoek in this mis- 
take, as he seems to have examined the lenses only of the carp and the perch, in which the two septa are most 
distinctly developed. 
f Elements of Natural Philosophy, vol. ii. p. 599. col. 1. and Plate XII. fig. 100. 
J Opera, vol. ii. p. 66. Lugd. Batav. 1722. 
