ON VISION. 363. 



When the attention is not directed to any particular object of sight, the 

 refractive powers of the eye are adapted to the formation of an image of 

 objects at a certain distance only, which is different in different individuals, 

 and also generally increases with increasing age. * Thus, if we open our 

 eyelids suddenly, without particular preparation, we find that distant 

 objects only appear as distinct as we are able to make them ; but by an 

 exertion of the will, the eye may be accommodated to the distinct percep- 

 tion of nearer objects, yet not of objects within certain limits. Between 

 the ages of 40 and 50, the refractive powers of the eye usually begin to 

 diminish, but it sometimes happens that where they are already too great, 

 the defect continues unaltered to an advanced age. It appears also that 

 after 50 or 60, the power of changing the focus of the eye is always much 

 impaired, amjtfstmietimes wholly lost. 



The mode, in which the accommodation of the eye to different distances 

 is effected, has long been a subject of investigation and dispute among 

 opticians and physiologists, but I apprehend that at present there is little 

 further room for doubting that the change is produced by an increase of 

 the convexity of the crystalline lens, arising from an internal cause. The 

 arguments in favour of this conclusion are of two kinds ; some of them 

 are negative, derived from the impossibility of imagining any other mode 

 of performing the accommodation, without exceeding the limits of the 

 actual dimensions of the eye, and from the examination of the eye in its 

 different states by several tests, capable of detecting any other changes if 

 they had existed : for example, by the application of water to the cornea, 

 which completely removes the effect of its convexity, without impairing 

 the power of altering the focus, and by holding the whole eye, when 

 turned inwards, in such a manner as to render any material alteration of 

 its length utterly impossible. Other arguments are deduced from positive 

 evidence of the change of form of the crystalline, furnished by the parti- 

 cular effects of refraction and aberration which are observable in the 

 different states of the eye ; effects which furnish a direct proof that the 

 figure of the lens must vary ; its surfaces, which are nearly spherical in 

 the quiescent form of the lens, assuming a different determinable curvature 

 when it is called into exertion. The objections which have been made to 

 this conclusion are founded only on the appearance of a slight alteration 

 of focal length in an eye from which the crystalline had been extracted ; 

 but the fact is neither sufficiently ascertained, nor was the apparent change 

 at all considerable : and even if it were proved that an eye without the 

 lens is capable of a certain small alteration, it would by no means follow 

 that it could undergo a change five times or ten times as great, t 



about 11 inches, the second spot disappears as though it had passed under a curtain : 

 on continuing to lift the head, the spot will reappear when the eye is .about 15 inches 

 from the paper. This was pointed out by Mariotte, Ph. Tr. 1G68, p. 668 ; 1670, 

 p. 1023. On the vanishing of images at points not coincident with the entrance of 

 the optic nerve, consult Brewster's Jour, of Sci. iii. 289. 



* On the effects of attention in vision see Purkinje, Beobachtungen zur Physio- 

 logic der Sinne, vol. i. Prag. 1823 ; vol. ii. Berlin, 1825. Heermann, Ueber die 

 BHdung der Gesichtsvorstellungen, Hanover, 1835. 



t Consult Pemberton, De Facultate Oc. ad Diversas Dist. se Acoommodandi, 

 Lug. Bat. 1719. Camper, De Oculo Humane, Lug. Bat. 1742. Albinus, Lug. 



2 A 



