258 
SIR DAVID BREWSTER ON THE CRYSTALLINE LENS. 
lens was now 0‘867 of an inch, and its thickness 0°733, being an increase of 0‘233 of 
an inch in thickness. 
On the seventh clay the capsule burst, and upon removing it and the soft pulp which 
formed about one tenth of an inch of the outer margin of the lens, the pink ring, 
with the white band both within and without it, and the black mass at the centre of 
the rectangular cross, were as distinct as ever. Hence it is manifest that the rise of 
the tint from yellow was not the effect of any expansive pressure produced by the 
swelling of the lens and the reaction of the capsule. 
The descent of the tint from bright blue to pink was no doubt owing to the po- 
larizing action of the extended capsule being withdrawn. In order to prove this I 
took the capsule, which is a tough and elastic membrane, and having stretched it, I 
found that it polarized, just before it tore, a white of the first order. Now the value 
of this tint is nearly equal to the difference between the values of the pink and blue 
of the second order of colours. 
The preceding results throw much light on the physiology of the crystalline lens ; 
and I shall have occasion, in a separate paper, to point out the conclusions to which 
they lead respecting the cause and cure of cataract. 
Allei'ly by Melrose , 
May 6th , 1837- 
