of the Capsule of the Crystalline Lens and Ciliary Zone, 1 1 



distances. Sir David Brewster* employs this argument suc- 

 cessfully against the hypothesis that the eyeball is lengthened, 

 or the convexity of the cornea increased, to accommodate the 

 eye to near objects. It is of equal force against the supposition 

 that the lens is drawn forward by any means; for this would 

 move the centre of the lens, and consequently the optical cen- 

 tre of the eye, forward, — an occurrence which the apparent 

 stability of stationary objects out of the axis of vision during 

 the adjustment, disproves beyond any doubt. 



In attempting to ascertain the seat and mechanism of a hid- 

 den function, as this is, it appears incumbent upon us not only 

 to show, as I have endeavoured to do, that a structure exists 

 which is capable of performing the function ; but also to prove 

 that the physical effects which the action of that mechanism 

 ought to produce, are actually produced in the living body, in 

 connexion with the exercise of the function itself. If this can 

 be done in the present case, all that the most cautious and ri- 

 gorous induction can demand, will, I venture to hope, be ful- 

 filled. 



In its natural state the crystalline lens appears to be a thin 

 gelatinous fluid, with a refractive power of about 1*377; for I 

 have repeatedly found it wholly so in the eye of the fcetus. It 

 is condensable even to solidity without destroying its transpa- 

 rency, and its refractive power keeps pace with its density, as 

 may be proved by suffering the fluid part of it to condense by 

 drying in a hollow prism made with two plates of glass. 



Now, if the capsule is the organ of adjustment, the lens, 

 from its condensability, ought to exhibit the following effects 

 of the capsular function: — 



1st. The contraction of the capsule ought, by pressing upon 

 its contents, to render them denser towards the centre than 

 towards the surface. For if the lens is a sphere consisting of 

 concentric layers or shells of equal thickness, it is obvious that 

 the pressure of the capsule, being propagated from the surface 

 to the centre, is equal over the whole of each layer. But as 

 the layers towards the centre occupy less space than those 

 which are towards the surface, the pressure on equal portions 

 must increase in the same proportion towards the centre; con- 

 sequently the central parts must in the course of time be ren- 

 dered denser by the reiterated action of the function, pressing 

 equal portions in the inverse duplicate ratio of the distance 

 from the centre. 



2nd. The more spherical the lens is, the denser, ceteris 

 paribus, ought its nucleus to be. For in a perfectly spherical 



* See Lardner's Cyclopaedia, Article Optics, p. 301. 

 C2 



