256 
SIR DAVID BREWSTER ON THE CHANGES IN THE STRUCTURE 
by the appearance of a brown tint in the dark neutral ring which separated the two 
positive structures. In the middle of the brownish black ring a trace of faint blueish 
light appears, generally in one of the sectors only, but gradually extends itself into a 
blue ring, which has negative double refraction and which is separated by distinctly 
formed black rings from the two positive structures, between which it lies. This 
state of the polarizing structure is shown in fig. 6, which is nearly the same as in the 
lens of the cow. 
The structure No. 1 , beginning at the centre, was pretty bright, but No. 3 was 
much more so, and No. 4 very faint, though perfectly distinct. 
On the second day the blue ring No. 2 was much enlarged, and had encroached 
greatly on the brightest structure No. 3, having reduced it both in breadth and in- 
tensity. No. 4 has also extended itself at the expense of No. 3. 
On the third day the new structure No. 2 had become the brightest of all. No. 4 
had increased also, whilst No. 1 had become smaller and fainter, and No. 3 was 
wholly obliterated. 
In another pair of lenses one of them burst at this stage of the development of the 
polarizing structures, while in the other the effect was singularly fine. No. 3 was 
wdiolly, and No. 1 nearly obliterated ; while the two new structures, which had no 
existence at first, were the only ones that remained. The new negative structure 
No. 2 consisted of four beautiful blue sectors of polarized light ; but in consequence 
of the great absorption of distilled water, and the consequent distension of the lens, 
it soon burst. 
I have already remarked that only one case has occurred in the course of my ex- 
periments in which the central structure of the lenses of quadrupeds was negative, as 
in fishes. In this case, however, the centre of the lens had its structure affected by 
some change in the condition of the fibres at their union in the three septa, which 
were not only distinctly seen, but had the polarizing structure clearly related to them. 
The polarized light filled up each of the three angles of 120° which lay between the 
three septa, and the intensity of the light was a maximum close to the three septa. 
Hence it is evident that the central negative structure was the result of an induration 
of the lens related to the septa, and had obliterated the positive structure which 
would otherwise have existed there. 
In examining the lenses of the horse I have observed the progressive development 
of its three structures as the animal advanced in age, and the extinction of all of 
them but one when the age of the animal was great. 
In both the lenses of a young horse three years old I found only one positive 
structure. 
In both the lenses of a horse whose age was unknown, I observed three structures 
beautifully developed. The central ones, which were extremely distinct and more beau- 
tiful in form and more intensely luminous than in any other quadruped which I had 
examined, were positive, the next structure negative, and the external on e positive. 
