268 SENSE OF SIGHT. 



view. The extract of belladonna has the power, when applied to the 

 eyelids, of dilating the pupil. A newly prepared article was thus ap- 

 plied, and in the space of about twenty minutes the pupil was so much 

 dilated, that the iris was almost invisible. From the time that preter- 

 natural dilatation occurred, objects, presented to this eye with the other 

 closed, were seen as through a cloud. The focus was found to be at 

 twice the distance of that of the sound organ; but, in proportion as the 

 effects of the belladonna passed off, and the pupil approached its natu- 

 ral size, vision became more and more distinct, and the focus nearer and 

 nearer the natural. In the open air, all objects except those near were 

 distinctly seen; but, on entering a room, all was enveloped in mist. 



There is, indeed, more evidence in favour of the utility of contraction 

 and dilatation of the pupil in distinct vision, within certain limits at 

 least, than of any of the other supposed methods of adjustment; and, 

 accordingly, the majority of opticians of the present day embrace this 

 view of the subject; but without being able to explain satisfactorily the 

 change in the interior of the eye effected by its movements. "It seems 

 difficult," says Sir David Brewster 1 one of the latest writers "to 

 avoid the conclusion, that the power of adjustment depends on the me- 

 chanism, which contracts and dilates the pupil; and as this adjustment 

 is independent of the variation of its aperture, it must be effected by 

 the parts in immediate contact with the base of the iris. By consider- 

 ing the various ways, in which the mechanism at the base of the iris 

 may produce the adjustment, it appears to be almost certain, that the 

 lens is removed from the retina by the contraction of the pupil." The 

 conclusion, drawn by Sir David, does not, however, impress us with the 

 same degree of certainty. 



Muller 2 thinks it most probable, that the faculty of the eye, which 

 enables it to adjust itself to different distances, depends on an organ, 

 which has a tendency to act by consent with the iris, but yet is in a cer- 

 tain degree independent of it. Reasoning per exclmionem, he thinks 

 it most probable, that the ciliary body has this motor power, and this 

 influence on the position of the lens; but admits, that we have no posi- 

 tive proof of its possessing contractility. More recently, however, 

 as has been shown, 3 the existence of a ciliary muscle has been de- 

 monstrated, which, by its contraction, may directly or indirectly advance 

 the lens. M. Pouillet, in his lectures before the Faculte des Sciences 

 of Paris, 4 explains the matter with no little confidence by the double 

 effect of the crystalline being composed of different layers, and the 

 mobility of the pupil ; a view, which had been previously maintained 

 in its essential characters by Treviranus. 5 As these layers are thinner 

 towards the axis of the crystalline than near its edges, by detaching 

 them successively the curvature of the remainder becomes greater and 

 greater, until the most central portion has the shape of a sphere. 

 Hence, such an apparatus will not have one focus only, but several, 



1 Op. citat, p. 252. a Elements of Physiology, by Baly, P. v. p. 1 150, June, 1839. 



3 Baly and Kirkes, Recent Advances in the Physiology of Motion, the Senses, &c., p. 24, 

 Lond., 1848. 



4 Siemens de Physique Experimental, t. iii. p. 331, Paris, J832. 



5 Beitriige zur Anatora. und Physiol. der Sinnenwerkzeuge, u. s. w., 1828. 



