244 OF SENSATION, AND THE ORGANS OF THE SENSES. 



posed membrane of Descemet, the other to the supposed continuation of that membrane over the anterior 

 surface of the iris; 7, choroid coat; 8, annulus albidus; 9, ciliary ligament; 10,10', ciliary body, consisting 

 of (10') a pars non-fimbriata, and (10) a pars fimbriata formed by the ciliary process; 11, ora serrata of the 

 ciliary body ; 12, iris; 13, pupil; 14, membrane of the pigment; 15, delicate membrane lining the posterior 

 chamber of the aqueous humour; 16, membrane of Jacob; 17, the optic nerve surrounded by its neurilema; 

 17', the fibres of the optic nerve consisting of fasciculi of primitive tubules ; 18, central artery of the 

 retina; 19, papilla cornica of the optic nerve ; 20, retina; the situation of its vascular layer is indicated by 

 a dotted line ; 21, central transparent point of the retina ; 22, vitreous humour; 23, the hyaloid membrane ; 

 24, canalis hyaloideus ; 25, zonula ciliaris; in the plate, none of its fimbriated part is seen, being concealed 

 by the ciliary processes; 26, canal of Petit; 27, crystalline lens; 28, anterior wall of the capsule of the 

 lens; 29, posterior wall of the capsule of the lens; 30, posterior chamber of the aqueous humour; 31, ante- 

 rior chamber of the aqueous humour.] 



are fixed upon an object, their axis must converge (as formerly explained, 

 254) so as to meet in it. The nearer the object, the greater must be the 

 degree of convergence ; and when the object is brought within the ordinary 

 distance of distinct vision, the convergence must very rapidly increase. Now 

 this is precisely what takes place, in regard to alterations in the focus of the 

 eye; for little change is required, when the object is made to approach from 

 a considerable distance to a moderate distance ; but, when it is brought near 

 the eye, the focus must be considerably lengthened, or the convexity of the 

 eye increased, to cause the rays to meet on the retina : and hence it may be 

 surmised, that the same cause is acting to produce both changes. But that 

 the convergence of the axis is not itself in any way the occasion of the altera- 

 tion of the focus of the eye, is shown by the fact, that the adaptation is as 

 perfect in a person who only possesses or uses one eye, as it is when both are 

 employed; and also by the power which is possessed by some persons of 

 altering the focus of the eye by an effort of the will, whilst the convergence 

 remains the same. In regard to the adaptation of the eyes to varying distances, 

 it is further to be remarked, that, when an object is being viewed as near to 

 the eye as it can be distinctly seen, the pupil contracts in a considerable 

 degree. The final cause of this change is evidently to exclude the outer rays 

 of the cone or pencil, which, from the large angle of their divergence, would 

 fall so obliquely on the convex surface of the eye, as to be much affected by 

 the spherical aberration; and to allow the central rays only to enter the eye, 

 so as to preserve the clearness of the image. The channel through which it 

 is effected is evidently the same as that by which the convergence of the eyes 

 is produced, namely, the inferior branch of the third pair of nerves ; to the 

 action of which, the sensations upon the retina form the stimulus, in the Same 

 manner as they do to the ordinary variation in the diameter of the pupil under 

 the influence of light. 



331. The ordinary forms of defective vision, which are known under the 

 names^of myopia and presbyopia, or short-sightedness and long-sightedness, 

 are entirely attributable to defects in the optical adaptation of the eye. In the 

 former, its refractive power is too great; the rays from objects at the usual 

 distance are consequently brought too soon to a focus, so as to cross one 

 another and diverge before they fall upon the retina; whilst the eye is 

 adapted to bring to their proper focus on the retina only those rays which 

 were previously diverging at a large angle, from an object in its near proxi- 

 mity. Hence a short-sighted person, whose shortest limit of distinct vision 

 is not above half that of a person of ordinary sight, can see minute objects 

 more clearly; his eyes having, in fact, the same magnifying power which 

 those of the other would possess, if aided by a convex-glass that would enable 

 him to see the object distinctly at the shorter distance. But as the myopic 

 structure of the eye incapacitates its possessor from seeing objects clearly, at 

 even a moderate distance, it is desirable, to apply a correction ; and this is 

 done by simply interposing a concave lens, of which the curvature is properly 



