HYPOTHESES IN EXPLANATION OF IT. 27 



of the remarkable property possessed by the eye of accommodating its 

 visual powers to the distinct perception of objects placed at such various 

 distances from it, the conclusion naturally forces itself upon the mind, 

 either that we still remain unacquainted with the real cause of this 

 singular property, or, which is more likely true, that it is the result of 

 two or more of the above-mentioned conditions acting together. For by 

 thus attributing to several of these conditions a certain relative share in 

 the production of this result, it is easy to perceive how by their united 

 agency, they may effect that alteration in the interior of the eye on which 

 the focal adjusting power of this organ depends, but which, individually, 

 it is improbable they could induce. 



The range of distance through which the human eye is capable of 

 adapting itself to distinct vision, has occupied the attention of Hueck and 

 Burow.* Both observers have found that this distance varies in different 

 individuals, whose sight in other respects may be quite perfect; and 

 Burow observes that, as a rule, there is a close correspondence between 

 the position of the most distant and that of the nearest point of vision, so 

 that the one being determined, the other may be inferred with tolerable 

 accuracy. The distance between the two bears a close relation also to 

 the refractive power of the eye, being less in proportion as the latter is 

 greater, and vice versa. According to Volkmann, the eyes of different 

 individuals likewise vary in their comparative refracting power over the 

 circumferential and central rays of light ; in some persons the former rays 

 are refracted more powerfully than the latter, while in others the reverse 

 is the case. A series of experiments conducted by Gruber, f led him to 

 the conclusion that even in the same individual, the two eyes have fre- 

 quently dissimilar distances for distinct vision, so that an object which 

 apparently is regarded by both eyes, is in reality distinctly seen only by 

 one, the focal distance of the other eye requiring to be altered, in order 

 that it likewise may have a clear sight of the object. 



In a case of singular congenital malformation of the iris in which the 

 transverse diameter of the pupil was considerably greater than the longi- 

 tudinal, the latter not exceeding in its middle or widest part, half a line, in 

 a moderate light, and the entire pupil completely closing in a strong light, 

 TourtualJ; found, among other peculiarities of vision, that objects having 

 a transverse direction could be perceived at a greater distance than those 

 which were longitudinal. This, and other circumstances, led him to the 

 conclusion that a transverse pupil will have the effect, like the opening 

 between the eyelids, of increasing the distinctness with which distant 

 objects are perceived. 



* Beitrage zur Physiol. und Physik des mensch. Auges, 1842. Noticed by Tourtual, 

 Loc. Cit. p. vii. 



t Canstatt's Jahresbericht. 1847, p. 194. J Miiller's Archiv. 1846, p. 346. 



