crystalline lens in fishes and quadrupeds . 31 7 
talline, but to the diameter which forms the axis of vision. 
For if the variation of density were related to the centre, the 
sphere would have a symmetrical structure, and like the glass 
ball already mentioned, would exhibit the same figure in every 
position. 
IV. It is highly probable, that this peculiar structure of the 
crystalline is necessary for correcting the spherical aberration. 
I have the honour to be, &c. 
D. BREWSTER. 
To the Right Hon. Sir Joseph Banks, Bart . G.C.B. P. R. S. 
Edinburgh, May 20, 1816. 
Tt£ 
