294 THOMAS YOUNU. 



Among the different modes of obtaining distinct images, 

 nature has assuredly made a choice, since man can see 

 with great distinctness at very different distances. The 

 question thus put has afforded a wide subject of remark 

 and discussion to physicists, and great names have figured 

 in the debate. 



Kepler and Descartes held that the whole ball of the 

 eye is susceptible of being elongated and flattened. 



Porterfield and Zinn contended that the crystalline 

 lens was movable ; and that it could place itself nearer 

 to, or further from the retina, as might be needed. 



Jurin and Musschenbroeck believed in a change in the 

 curvature of the cornea. 



Sauvages and Bourdelot supposed also that a change 

 in curvature took place, but only in the crystalline lens. 

 Such is also the system of Young. Two memoirs which 

 our colleague successively submitted to the Royal Society 

 of London include the complete development of his views. 



In the first of these, the question is treated almost 

 entirely in an anatomical point of view. Young there 

 demonstrates by the aid of direct observations of a very 

 delicate kind, that the crystalline is endowed with a 

 fibrous or muscular constitution, admirably adapted to 

 all sorts of changes of form. This discovery overthrew 

 the only solid objection which had, till then, opposed the 

 hypothesis of Sauvages and Bourdelot. 



That hypothesis had no sooner been announced than it 

 had been attacked by Hunter. 



Thus this celebrated anatomist aided the cause of the 

 young experimenter by the attention drawn to the sub- 

 ject, while his labours were as yet unpublislied, and not 

 even communicated to any one. Howevei", this point of 

 the discussion soon lost its importance. The learned 



