46 
SIR DAVID BREWSTER ON THE ANATOMICAL AND OPTICAL 
^ 5. On the Structure of the Crystalline Lens of the Turtle and other animals in which 
the Fibres are differently combined in the anterior and posterior Surfaces. 
In the various structures of the crystalline lens which have been described in pre- 
ceding- communications, the fibres are similarly arranged on both the surfaces of the 
lens, whether these two surfaces are similar, or have different degrees of convexity. 
In this respect they resemble the artificial lenses of the optician, in which there is no 
other deviation from symmetry but in the curvature of their surfaces. I have disco- 
vered, however, in the lens of the turtle, and in that of several fishes, a new combina- 
tion of fibres, in which they are differently arranged in the anterior and posterior 
faces of the lens. 
Ever rich in her forms and fertile in her resources. Nature thus presents to us in 
the crystalline lens four singular properties, which the most skilful optician, even if 
he knew their design, is not likely ever to attempt to imitate. But the study of 
these properties is not on this account the less interesting ; for though we may never 
be able to produce the same effect, either by similar or analogous means, yet we may 
be led to discover some other principle within the sphere of art by which the desired 
result may be obtained. The four properties to which I refer are the increase of 
density from the surface to the centre of the lens ; the alternations of negative and 
positive structures, as exhibited by the action of the lens on polarized light ; the 
arrangement of the fibres in reference to different numbers of septa ; and the defect 
of symmetry in this arrangement in the turtle and a few fishes. The first of these 
properties, namely, a variation of density, is no doubt intended to correct spherical 
aberration, an effect which may be produced by the union of several spherical sur- 
faces, or by hyperbolical or elliptical surfaces, or by surfaces of contrary flexure ; 
but the design of the other three properties has not even excited the ingenuity of 
conjecture, and will probably remain among the numerous problems which will exer- 
cise the sagacity of another age. 
When I first observed a defect of symmetry in the arrangement of the fibres in the 
two halves of the lens of the turtle, I was extremely doubtful of the accuracy of my 
observations, and was therefore at peculiar pains to confirm the result by examining 
several lenses of the turtle. In every lens, however, I found the deviation from sym- 
metry was clearly indicated, though it did not possess the same character in every 
lens which came under my notice. 
In the eye of the turtle which I first examined, the ball was one inch in diameter, 
and the diameter of the lens only 0'200 of an inch. The lens was nearly spherical ; 
and it had on its anterior face two septa, like the hare and the salmon, as shown in 
Plate VI. fig. 1 ; but on its posterior face the fibres converged to a single pole, as 
shown in fig. 2. In this structure there are only four fibres in each lamina, which have 
their different parts lying in the same plane. All the other fibres are concave towards 
a plane passing through the two anterior septa, and of course convex towards a plane 
