TRAXSACTIOXS OF THE SECTIONS. 15 



ferent distances of the lens from the retina different diameters of it will 

 be placed in the axis of vision. As the diameters of a sphere are all 

 equal and similar, Mr. Clay Wallace considered that vision would be 

 equally perfect along the different diameters of the lens, brought by 

 rotation into the axis of vision. Sir David Brewster, however, remarked 

 that he had never found among his numerous examinations of the lenses 

 of fishes any which are perfectly spherical, as they were all either 

 oblate ov prolate spheroids, so that along the different diameters of the 

 solid lens the vision would not be similarly performed. But, inde- 

 pendent of this circumstance, he stated that in every solid lens there 

 was only one line or axis in which vision could be perfectly distinct, 

 namely, the axis of the optical figure, or series o^ positive and negative 

 luminous sectors, which are seen by the analysis of polarized light. 

 Along every other diameter the optical action of the lens is not sym- 

 metrical. When the lens is not a sphere, but lenticular, as in the 

 human eye or in the eyes of most quadrupeds, Mr. Clay Wallace con- 

 siders that the apparatus for adjustment is the ciliary processes, to 

 which this office had been previously ascribed, though not on the same 

 scientific grounds as those discovered by Mr. Wallace. One of the 

 most important results of Mr. Wallace's dissections is the discovery of 

 Jibres in the retina. These fibres may be rendered distinctly visible. 

 They diverge from the base of the optic nerve, and surround the^ra- 

 men ovale of Soemmerring at the extremity of the eye. Sir John Her- 

 schel had supposed such fibres to be requisite in the explanation of 

 the theory of vision, and it is therefore doubly interesting to find that 

 they have been actually discovered. 



On the Structure of the Vitreous Humour of the Eye of a Shark. 

 By Sir J. W. F. Herschel, Bart. 



Sir J. Herschel states, that while crossing the Atlantic on his return 

 from the Cape, a shark was caught in lat. 2° N. and long, about 20° W. 

 Having procured the eyes, which were very large, and extracted the 

 crystalline lenses, the vitreous humour of each, in its capsule, presented 

 the usual appearance of a very clear, transparent, gelatinous mass, of 

 little consistency, but yet forming, very distinctly, a connected and 

 continuous body, easily separable from every other part. Wishing to 

 examine it more narrowly, it was laid to drain on blotting-paper ; and, 

 as this grew saturated, more was applied, till it became apparent that 

 the supply of watery liquid was much too great to be accounted for by 

 adhering water or aqueous humour. " Becoming curious to know to 

 what extent the drainage might go, and expecting to find that, by car- 

 rying it to its limit, a gelatinous principle of much higher consistency 

 might be insulated, I pierced it in various directions with a pointed in- 

 strument. At every thrust a flow of liquid, somewhat ropy, but de- 

 cidedly not gelatinous, emanated ; and, by suspending it on a fork, and 

 stabbing it in all directions with another, this liquid flowed so abun- 



