TRANSACTIONS. 



I. — On a Posi'ble Explanation of the Adaptation of the Eye to Distinct Vision at 

 Different Distances. By Sxuys, D. Forbes, Esq., F.R.SS.L.^ E., Corre- 

 sponding Member of the Institute of France, and Professor of Natural Philo- 

 sophy in the University of Edinburgh. 



[Read 16th December 1S44, and 6tli January 1845.] 



It is unnecessary to detail to this Society tlie 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 heen so circumstantially and correctly recapitulated in systematic works 

 (as for instance in Young's Lectures and in Muller'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 



VOL. XVI. P.iET I. A 



