ADAPTATION OF THE EYE TO DISTANCES. 269 



as many, in fact, as there are superposed layers ; the foci being nearer 

 and nearer as we approach the central spherical portion. This arrange- 

 ment, he says, enables us to see at all distances, inasmuch as, "having 

 an infinite number of foci at our disposal, we can use the focus that suits 

 the object we are desirous of viewing." If, for example, it be a near 

 object, the pupil contracts, so as to allow the rays to fall only on the 

 central parts; if more distant, the pupil is dilated to permit the rays to 

 pass through a part that has a more distant focus. 



It is obvious, however, that in such a case, the ordinary inconve- 

 nience of the aberration of sphericity must result; for when the pupil 

 is dilated, the rays must pass through the more marginal, as well as 

 the central part of the lens. M. Pouillet was aware of this difficulty, 

 but he has not disposed of it philosophically. "It may be said that in 

 opening the pupil widely, the light is not precluded from passing by 

 the centre, and that a kind of curtain would be required to cover the 

 part of the lens, which is unemployed. To this I reply, that there is 

 no necessity to prevent the rays from passing by the axis of the crys- 

 talline ; for what is the light, which passes through this small space, 

 compared with that which passes through the great zone of the crystal- 

 line? It may be looked upon as null." 



It must be admitted, with M. Longet, 1 that if the fact of the adapta- 

 tion of the eye to vision at different distances be received as incon- 

 testable, the mechanism of the phenomenon must be regarded as entirely 

 unknown; not one of the explanations offered being able to carry 

 conviction. The whole affair is, indeed, enveloped in perplexity, and 

 it is rendered not less so by the fact mentioned by M. Magendie, that if 

 we take the eye of an albino animal, and direct it towards a luminous 

 object, we find a perfect image depicted on the retina, whatever may 

 be the distance of the object; the image, of course, being smaller and 

 less luminous when remote, but always distinct. Yet, in this experi- 

 ment, the eye being dead, there can be neither contraction nor dila- 

 tation of the pupil. This result has induced Magendie 3 and not too 

 hastily, we think to draw the conclusion, that although theory may 

 suggest, that there ought to be such adaptation as has been presumed 

 and attempted to be accounted for, observation proves, that such may 

 not be the case; and, consequently, all the speculations on the subject, 

 however ingenious they may be, must fall to the ground. Dr. Fletcher, 

 too, after alluding to the various hypotheses on the subject, adds: 

 "It appears absurd to attempt to explain a fact which has no real 

 existence, since it has never been proved that the eyeball has any ca- 

 pability of adapting itself to different distances, or that any such adap- 

 tation is required." 3 We are, indeed, not justified, perhaps, in admit- 

 ting more than a slight accommodation from the contraction of the 

 pupil in viewing near objects effected in the mode already explained. 

 If the accommodation existed to any material extent, it is difficult to 

 understand, why minor degrees of short or long-sightedness should not 



1 Traite de Physiologie, ii. 70, Paris, 1850. 



2 Precis Elementaire, i. 72. 



3 Rudiments of Physiology, Part iii. p. 48, Edinburgh, 1837. 



