2 PROFESSOR FORBES ON THE ADAPTATION OF THE EYE 



form an optically distinct picture on the retina ; and that the opinion of those 

 physiologists is to be disregarded, who have supposed that the distinctness of 

 vision at one distance or another arises merely from a mental effort of attention. 



We assume it, then, to be granted that the adjustment of which we are in 

 quest is of a nature such, that when the eye is turned from a distant to a near 

 object, either the retina is moved from the refracting apparatus of the ej^e, so that 

 the less convergency of the rays may be allowed for by the increased distance ; 

 or else, that the distance of the retina or pictured screen remaining the same, the 

 refraction of the eye is increased, so as to cause the rays to converge more rapidly 

 than they would have done in the previous state of the eye. 



If the first alternative be true, the axis of the eye must undergo an elongation 

 of about one-seventh part (according to Olbers* and YouNcf ), in passing from 

 the distinct vision of distant to that of very near objects. Dr Young has described 

 two experiments, by which, he says, he satisfied himself that the elongation of 

 the eye could not be anjiihing like the quantity required by the hypothesis. Dr 

 Young's experiments are obscurely described ; but perhaps a not less conclusive 

 and more simple proof of the error of this explanation is found in the fact insisted 

 on by Treviranus and Muller, and which seems to me quite unanswerable, — 

 " that the tendency of the straight {recti) muscles is merely to retract the eye, 

 and if resistance were afforded by the cushion of fat behind it, to flatten rather 

 than elongate it ; their action would, therefore, have the effect of adapting the 

 eye to the vision of distant objects only, the image of which is formed nearer the 

 lens, than that of near objects ; while it is in looking at very near objects, on the 

 contrary, that we are conscious of an effort Avithin the orbit." :j: 



It seems difficult to admit with Muller, however, that any conclusion as to 

 the mechanism of the eye can be drawn from the transient and anomalous changes 

 ofadjustment which it seems to undergo under the influence of narcotics, such as 

 belladonna. 



The other class of explanations turn upon the production of an increased 

 refractive power in the eye, by the altered curvatm-e of one of its numerous re- 

 fracting surfaces. Every one of these has been in turn fixed upon as the subject 

 of the change, as well as those parts of structure which, by their intimate con- 

 nexion with the principal parts, might be supposed to influence them. The 

 cornea, the lens, the fris, and the ciliary processes, have each been supposed to 

 be the part immediately affected. Most of the theories have been refuted with 

 consummate skiU by Dr Young, in his paper on this subject in the Philosophical 

 Transactions for 1800 ; and, as is well known, he himself attributed the change 

 of focal adjustment to a proper muscular power residing in the lens. This other- 



* Quoted by MUller. t Nat. Phil. ii. 589. 



+ Mulleb's Physiology, translated, p. 1143, 1144. 



