410 ON SENSATION, AND THE ORGANS OF THE SENSES. 



supposed 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 ihe 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 neurilemma; 17', the fibres of the optic nerve consisting of fasciculi of primitive! ubules ; 18, cen- 

 tral 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 j 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, an- 

 terior wall of the capsule of the lens ; 29, posterior wall of the capsule of the lens ; 30, posterior chamber 

 of the aqueous humour ; 31, anterior chamber of the aqueous humour.] 



ordinarily read), the rays proceeding from an 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 contrary, the rays from an 

 object at a greater distance would meet before they 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 adapta- 

 tion 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 converg- 

 ing 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 distance of the object. 

 It is not improbable, that both of these methods are employed in the Eye, 

 though no distinct evidence has been obtained of the operation of either. Seve- 

 ral hypotheses have been proposed, to account for the phenomenon : it is 

 easily proved that no one of them can alone be true ; but it cannot be readily 

 shown that any of them is entirely false : and it would not seem unlikely, 

 therefore, that all may participate, in various degrees, in the effect. The fol- 

 lowing are the principal of these. 1. An alteration in the form of the globe 

 of the eye by the action of the muscles, so that its antero-posterior diameter 

 may be increased or diminished.* 2. A change in the convexity of the cor- 

 nea. This might be very well connected with the last; since, if the globe 

 were converted into a spheroid, of which the antero-posterior diameter would 

 be the longest, the curvature of the cornea would be increased ; whilst, if the 

 antero-posterior diameter were shortened, the curvature would be diminished. 



3. Change of position of the crystalline lens, by means of the ciliary processes. 



4. Change of figure of the lens itself. That one or both of the last two are 

 concerned in the effect, would appear from the fact, well known to every Oculist, 

 that, after the removal of a cataract, the power of adapting the eye to dis- 

 tances is greatly diminished. 5. Change in the aperture of the pupil ; the 

 mode in which this could assist in accommodating the eye to variations of 

 distance, is not very obvious. 



537. Some curious circumstances, relative to the connection between the 

 optical adaptation of the eye to distances, and the changes in the direction of the 

 axes of the two eyes, have been pointed out by Miiller. When both eyes are 

 fixed upon an object, their axes must converge (as formerly explained, 455) 

 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 considera- 

 ble distance to a moderate distance; but, when it is brought near the eye, the 



* The influence of the muscles in altering the form of the globe may be better compre- 

 hended, now that we know the mode in which this is kept in its place in the front of the 

 orbit, by a fascia passing behind, it, and attached anteriorly to the lids. 



