TRANSACTIONS OF THE SECTIONS. Al 
On the cause of the Optical Phenomena which take place in the 
Crystalline Lens during the absorption of Distilled Water. By 
Sir D. Brewster, K.H., &e. 
Sir David Brewster commenced by drawing the attention of the 
Section to a representation of the eye of the sheep found among the 
MSS. of Sir Isaac Newton, in the possession of Lord Portsmouth. The 
several parts of the drawing under consideration were most care- 
fully laid down on one scale, and the exact measurements given, re- 
specting the cornea in particular. It appeared that it was a portion of 
an ellipsoid, somewhat longer, but not so deep as the ball of the eye, 
the cornea being a portion of its most convex part at the major axis. Sir 
David then went on to introduce the subject of the present communi- 
cation, by briefly running through the leading points to which he had 
adverted at the last meeting of the Association, regarding a series of 
experiments on the crystalline lenses of quadrupeds. From these it 
appeared that the capsule of the lens absorbs water with great avidity ; 
and during this process exhibits (when exposed to the analysis of 
polarized light) remarkable changes both in the nature and in the 
number of the positive and negative doubly refracting structures of 
which it is composed. These singular, and, in the case of the lens of 
the horse, very beautiful phenomena, Sir David stated that he was 
not able to explain when he first made the communication ; but he had 
_ Since returned to the subject, and had succeeded in discovering the cause 
_ Of the various phenomena which he had observed. While the capsule 
_ of the lens is absorbing distilled water, the bulk of the lens is gradually 
increasing, and consequently the capsule, which he found to be highly 
elastic, became more and more stretched in the direction of the radii of 
its circular margin. This extension produces, as may be shown by di- 
rect experiment, a negative doubly refracting structure, like the central 
portion of a positive system of polarized rings, with a rectangular black 
cross. The tint of this membrane rises to a white of the first order; 
and, as the membrane is double, the two tints will produce, when 
- combined, a purple of the first order, which will be the maximum tint 
_ developed by the extended capsule just before it bursts. Now it is 
_ obvious that the optical figure thus given by the capsule alone will, 
when combined with the fixed optical figure of the lens itself, produce 
all the variable phenomena previously observed. If the fixed optical 
figure consist of two structures, both positive, then one part of the 
capsule will produce, in the neutral black ring, a negative doubly re- 
fracting luminous ring, which separates the two positive luminous 
_ rings; while the outer and inner portions of the capsule will act in op- 
position to the positive structures of the lens, and tend to diminish or 
obliterate the tints produced at these parts. The result of this com- 
bination of actions will be the production of a certain optical figure, in 
which a negative series of luminous sectors is placed between two 
positive series of luminous sectors. In the process by which these 
_ changes are produced, a new series of luminous sectors, having ne- 
_ gative double refraction, has been made to appear in the centre of the 
