12 Mr. T. Smith on the Muscular Structure and Functions 



lens, the whole pressure of the capsule is sustained by a single 

 central particle. But in a flat lens the same pressure is sus- 

 tained by a number of particles occupying an extended space. 

 If the force of pressure is the same in all lenses, the density, 

 at the centre of a flat lens, ought to be the same as it is found 

 to be at the same distance from the surface of a spherical 

 lens. 



3rd. The more the eye has been employed in surveying 

 near objects, the harder ought its central nucleus to be. Hence 

 the central density ought to increase with the age of the ani- 

 mal; and it should be found, ceteris paribus, harder in indivi- 

 duals that are much employed in near vision than in those of 

 a contrary habit. Even short-sightedness may, in some in- 

 stances, be expected as the consequence of long intense adap- 

 tation to near objects. 



4th. The hardest part of the crystalline ought to be in the 

 centre of pressure. This is an evident and important corol- 

 lary of the theory ; and as the convexity of the two surfaces of 

 the lens is seldom or never equal, the position of the hardest 

 point of the nucleus will furnish an excellent test of its truth. 

 If the capsule is the organ of adjustment, the distance of the 

 hardest point from the anterior surface will be to its distance 

 from the posterior, as the radius of curvature of the former is 

 to the radius of curvature of the latter. For there the centre 

 of pressure of the two surfaces must be. 



5th. The indurated nucleus of the unadjusted lens ought 

 to be more spherical than the soft external part. For when 

 the central parts have gained that degree of firmness and con- 

 sistence that enables them to retain any figure given to them, 

 the figure which they will assume will obviously be determined 

 by the figure of the contracted capsule adapting the eye to the 

 nearest distance. If the external soft parts are sufficiently 

 abundant, this will enable the capsule to adapt the eye to di- 

 stant objects by moulding the soft parts, while the indurated 

 nucleus retains the convexity given to it by the contracted 

 capsule. This therefore will furnish another important test 

 by which the truth of the theory may be tried; for no other 

 cause that I am aware of could produce the same effect. 



It now remains to ascertain, by examination of the lens 

 itself, whether or not its structure and appearances correspond 

 with the theory in these respects. 



1st. The greater density of the crystalline at the centre than 

 at the surface has long been known, and is universally acknow- 

 ledged by physiologists. Sir David Brewster demonstrates in 

 an elegant manner its gradual increase of density from the 

 surface to the centre. Upon exposing a recent human crystal- 



