of the Capsule of the Crystalline hens and Ciliary Zone, 15 



point is difficult to be determined by the method I have men- 

 tioned. I regret that it is not in my power to try the question 

 by means of polarized light ; for it would be highly interesting 

 to know whether the want of symmetry of structure of the cry- 

 stalline lens of fishes, observed by Sir David Brewster on ex- 

 posing it in different positions to polarized light, was owing 

 to the hardest or densest part lying nearer to the more con- 

 vex surface than to the other, and whether the ratio of the 

 difference could be found in that way. 



5th. The figure of the nucleus corresponds admirably with 

 the theory, and if carefully attended to it may enable the intel- 

 ligent observer, who has full opportunities of examining the 

 human lens, to ascertain, by comparing the whole lens and its 

 nucleus in different eyes, the degree of sphericity which the 

 functional contraction of the capsule can produce, Haller is 

 the only author that I know of, who has noticed the greater 

 sphericity of the nucleus than of the whole lens. That dis- 

 tinguished physiologist mentions having found the nucleus of 

 the badger quite spherical, though the whole lens itself was not 

 so*. Notwithstanding the silence of other authors, the fact is 

 very evident, and may be easily demonstrated as follows : — 

 Deprive the lens of its capsule and plunge it in boiling water: 

 when it has become opake and firm, take it out of the water 

 and divide it into two equal parts, without deranging its curva- 

 tures, by a section along the axis. The greater sphericity of 

 the nucleus than of the whole lens will then appear in the 

 most distinct manner ; for the change it undergoes in boiling 

 water produces no sensible effect on its convexity. 



It is an interesting fact, that the figure which the whole lens 

 acquires by contraction of the capsule in boiling water, is al- 

 most or altogether the same as that of the nucleus in its na- 

 tural state. In the lenses of several old cows, I have observed 

 a very remarkable circumstance. When I first observed that 

 the ratio of the diameter to the thickness of the whole lens 

 in its natural state, was different from the ratio of the diameter 

 to the thickness of the nucleus, I examined a great number 

 of lenses for the sake of ascertaining whether the fact was uni- 

 versal or only accidental. The result was, that the central part 

 of the nucleus was always decidedly more spherical than the 

 whole lens ; but in some old lenses, I observed that the ex- 

 ternal laminae of the nucleus approached nearer and nearer 

 to the figure of the whole lens; from which it follows, that in 

 advanced age, the force by which the figure of the nucleus is 

 determined, is, cceteris paribus, less than in youth. This fact, 



' • Physiologic*, lib. xvi. sect. 2. 



