TRANSACTIONS. 

* 
I.—On a Possible Explanation of the Adaptation of the Eye to Distinct Vision at 
Different Distances. By James D. Fores, Hsq., &.R.SSL. § E., Corre- 
sponding Member of the Institute of France, and Professor of Natural Philo- 
sophy in the University of Edinburgh. 
[Read 16th December 1844, and 6th January 1845. ] 
Iv is unnecessary to detail to this Society the various ingenious hypotheses 
which have been proposed to account physiologically for the accommodation of the 
eye to distinct vision at different distances. In later years, these different theories 
have been so circumstantially and correctly recapitulated in systematic works 
(as for instance in Youne’s Lectures and in MUtLER’s Physiology), that it would 
be a waste of time to copy and recite them here. I will only do so, then, so far as 
may be necessary to justify the attempt I have now to make, and to strengthen 
my views by those of others, as far as they bear upon them. 
The eye being the organ of sense best understood, and constructed upon the 
most intelligible principles,—being one whose functions, up to a certain point, may 
be accurately represented by an artificial apparatus, it is impossible to doubt that 
the ultimate function of vision depends on the formation of a distinct picture of 
an object upon the retina, and that the circumstances which affect the distinctness 
of the picture in the instrument or artificial eye, must affect the clearness of 
vision in the real eye. Such a circumstance is notoriously the distance at which 
objects are placed from the eye. Now it is known by experience, (1.) That ob- 
jects at very variable distances may, in the healthy organ, be distinctly seen ; 
(2.) That such variations have limits, beyond which distinct vision cannot, by any 
effort, be obtained; (3.) This limit varies in different eyes ; (4.) The limit may be 
extended by optical aid, which would, in the model or artificial eye, produce the 
same effect; (5.) The adjustment of the eye to different distances is felt to be 
accompanied by a distinct muscular effort. On all these grounds, we conclude 
that the focal adjustment of the eye is a real mechanical adjustment, tending to 
VOU, XVi. PART I. A 
