ov VISION. 451 



vexity of the crystalline ]e"s, arising from an internal cause. The argu- 

 ments in favour of this conclusion are of two kinds; some of them are nega- 

 tive, derived from the impossibility of imagining any other mode of perform- 

 ing the accommodation, without exceeding the limits of the actual dimen- 

 sions 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 co!r.r)lctely re- 

 moves the effect of its convexity, without impairing the power of altering the 

 focuSj'wand by holding the whole eye, when turned inwards, in sucl^ a 

 manner as to render any material alteration of its length utterly impossible. 

 Other arguments arc deduced from positive evidence of the change of form 

 of the crystalline, furnislied by the particular effects of refraction and aber- 

 ration 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 pro^ved 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. 



The iris serves, by its variable magnitude, to exclude more or less of the 

 light falling on the cornea, when its intensity would otherwise be too great; 

 hence the pupil is usually smallest by day, and its increased magnitude at 

 night sometimes gives the eye a greater apparent lustre. The iris also in- 

 tercepts such rays as would fall on parts incapable of refracting them regu- 

 larly; and by its contraction when a nearer object is viewed, it lessens the 

 confusion which would arise, in such eves as cannot accommodate them- 



7.4/ 



selves sufficiently, from the magnitude of the imperfect focal points on the 

 retina. Such a contraction almost always accompanies the diminution of 

 the focal length, even in a perfect eye, and it may easily be rendered visible 

 by walking gradually up to a looking glass, and observing the magnitude 

 of the pupil as we approach nearer and nearer to our image. It would be 

 difficult to assign a reason for this change of the state of the pupil within the 



