196 Sir. David Brewster on the Changes in the Structure 



stinctly 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 intensity. 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 wholly, 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 experiments 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 struc- 

 ture 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 pro- 

 gressive development of its three structures as the animal ad- 

 vanced 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 ob- 

 served three structures beautifully developed. The central ones, 

 which were extremely distinct and more beautiful in form and 

 more intensely luminous than in any other quadruped which I 



