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, ether the retina is moved from the refracting apparatus of the eye, 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 Youne+), in passing from 
the distinct vision of distant to that of very near objects. Dr Youne has described 
two experiments, by which, he says, he satisfied himself that the elongation of 
the eye could not be anything like the quantity required by the hypothesis. Dr 
Youne’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 MULtEr, and which seems to me quite unanswerable,— 
“that the tendency of the straight (vect?) 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 within the orbit.” t 
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 
of.adjustment 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 curvature 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 iris, and the ciliary processes, have each been supposed to 
be the part immediately affected. Most of the theories have been refuted with 
consummate skill by Dr Youne, 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 Mixzer. t Nat. Phil. ii. 589. 
+ Muxtzr’s Physiology, translated, p. 1148, 1144. 
