ADAPTATION OF THE EYE TO DISTANCES. 267 



ceding writers. The lens, he says, is composed of coats more firm and 

 tenacious, as well as more refractive towards the centre, and less so at 

 the sides. These coats are also nearly spherical in the centre, forming 

 a nucleus of considerable resistance. Hence he supposes, that if the 

 lens be compressed in any manner by a uniform hydrostatic pressure, 

 it will yield more readily in a plane at right angles to the axis of vision ; 

 and the lens will become more spheroidal, and consequently more re- 

 fractive, that is, adapted for the vision of objects at small distances. 

 This hydrostatic pressure is believed to be conveyed from the humours 

 of the eye, between which the lens is delicately suspended, and to ori- 

 ginate in the action of the muscles that move the eyeball compressing 

 simultaneously the tough sclerotic coat. 



It is somewhat singular, that on a subject where so many opportuni- 

 ties have occurred for establishing the fact definitively, such difference 

 of opinion should exist regarding the question, whether an eye from 

 which the crystalline has been removed, as in the operation for cataract, 

 be capable of adjusting itself to near objects. Haller 1 and Knox, 

 amongst others, decide the question affirmatively; Porterfield, Young, 

 and Travers, 2 negatively. M. Magendie, as we have seen, considers the 

 great use of the crystalline to be, to increase the brightness and sharp- 

 ness of the image by diminishing its size. Mr. Travers again, regards 

 the adjustment as a change of figure in the lens; not, however, from a 

 contractile power in the part itself, but in consequence of the lamellae, 

 of which it is composed, sliding over each other, when acted upon by 

 external pressure; whilst upon the removal of this pressure, its elastic 

 nature restores it to its former sphericity. The iris is conceived to be 

 the agent in this process; the pupillary part of the organ being, in the 

 opinion of Mr. Travers, a proper sphincter muscle, which, when it con- 

 tracts and relaxes, tends, by the intervention of the ciliary processes, 

 to effect a change in the figure of the lens, which produces a correspond- 

 ing change in its refractive powers. 



3. One of the causes to which the faculty of seeing at different dis- 

 tances has been ascribed is the contraction and dilatation of the pupil. 

 It has been already observed, that when we look at near objects, the 

 pupil contracts, so that the 'most divergent rays do not penetrate the 

 pupil; and vision is distinct. Hence, it has been conceived probable, 

 by De La Hire, 3 Haller, 4 and others, that the adjustment of the eye to 

 various distances within the limits of distinct vision may be effected by 

 this mechanism, in the same manner as it regulates the quantity of light 

 admitted into the organ. Certain it is, that if we look at a row of 

 minute objects, extending from the visual point outwards, the pupil is 

 seen to dilate gradually as the axis of the eye recedes from the nearest 

 object. 



An experiment made by the author, on his own eye, 5 when a student 

 of medicine, has been quoted by Dr. Fleming 6 as confirmatory of this 



1 Element. Physiol., lib. xvi. sect. 4. 



* A Synopsis of the Diseases of the Eye, Lond., 1824. 



3 Memoir, de 1'Acad. des Sciences de Paris, torn. ix. p. 620. 



4 Element. Physiol., torn. v. lib. xvi., 4. 



* Annals of Philosophy, x. 432. 6 Philosophy of Zoology, i. 187, Edinb., 1822. 



