SENSE OP VISION. 875 



object placed Nearer to the eye than this would not be brought to a focus upon 

 the retina, but would converge towards a point behind it ; whilst, on the con- 

 trary, the rays from an object at a greater distance would meet before they 



Fig. 208. 



A longitudinal section of the globe of the Eye : 1, the sclerotic, thicker behind than in front; 2, the cornea, 

 received within the anterior margin of the sclerotic, and connected with it by means of a bevelled edge ; 3, the 

 choroid connected anteriorly with (4) the ciliary ligament, and (5) the ciliary processes ; 6, the iris ; 7, the 

 pupil ; 8, the third layer of the eye, the retina terminating anteriorly by an abrupt border at the commence- 

 ment of the ciliary processes ; 9, the canal of Petit, which encircles the lens (12); the thin layer in front of 

 this canal is the zonula ciliaris, a prolongation of the vascular layer of the retina to the lens ; 10, the anterior 

 chamber of the eye, containing the aqueous humor ; the lining membrane by which the humor is secreted is 

 represented in the diagram ; 11, the posterior ; 12, the lens more convex behind than before, and inclosed in 

 its proper capsule; 13, the vitreous humor inclosed in the hyaloid membrane, and in cells formed in its inte- 

 rior by that membrane : 14, a tubular sheath of the hyaloid membrane, which serves for the passage of the 

 artery of the capsule of the lens; 15, neurilemma of the optic nerve; 16, the arteria centralis retinae, imbed- 

 ded in its centre. 



reached the retina, and would have again diverged from each other when they 

 impinge upon it ; so that, in either case, vision would be indistinct. Now, two 

 methods of adaptation suggest themselves to the Optician. Either he may 

 vary the distance between the refracting surface and the screen on which the 

 image is formed, in such a manner that the latter shall always be in the focus 

 of the converging rays ; or the distance of the screen remaining the same, he 

 may vary the convexity of his lens in such a manner as to adapt it to the dis- 

 tance of the object. The mode in which this adaptation is effected in the Hu- 

 man Eye has not yet been clearly made out ; and many hypotheses have been 

 put forward respecting it. According to the calculations of Olbers, based on 

 the ascertained refractive powers of the media of the eye, the difference be- 

 tween the focal distances of the images of two objects, the one so far off that 

 its rays are parallel, and the other at the distance of only four inches from the 

 eye, is about 0.143, or one-seventh of an inch ; but, as the usual range of dis- 

 tinct vision does not extend to objects brought within six or seven inches, the 

 amount of change required in the relative places of the refracting bodies and 

 the retina would not ordinarily exceed a line. It has been thought that this 

 change might be produced by an alteration in the convexity of the cornea, or 

 by an elongation of the globe of the eye generally, or by both methods in com- 

 bination j which alterations, it was supposed, might be effected by the action of 

 the muscles of the eyeball. But no such changes have been detected by the 

 most careful measurement ; and it cannot be shown how any contractile action 

 of the muscles of the eyeball could produce an elongation of the eye, since 

 their tendency would be (when acting altogether) to draw it backwards into its 

 socket, or, this being prevented by the fascia and cushion of fat against which 



