222 Biographical Memoir of Dr Thomas Young. 



the lens, and a contrary movement becomes necessary if the ob- 

 jects remove farther off. To preserve to the images, then, all 

 the necessary distinctness, without changing the position of the 

 surface which receives them, is impossible, unless the curve of a 

 lens varies ; that increases when we look at near objects, and di- 

 minishes when we look at distant ones. 



Amongst the various methods of attaining distinct images, na- 

 ture has necessarily made choice of one; for man can see with the 

 greatest accuracy at distances which are very different. The 

 question thus introduced has been a subject of keen research 

 and discussion among philosophers, and many illustrious men 

 have taken part in the debate. 



Kepler, Descartes, and many others, have maintained that the 

 whole of the globe of the eye is susceptible of being flattened 

 and elongated. 



Forterjield, Zinn, and others, have supposed that the crystal- 

 line was moveable, and, as required, could move nearer to or far- 

 ther from the retina. 



Jurin, Miischenhroek, and others, thought that there was a 

 change in the curvature of the cornea ; while Sauvages, Bour- 

 delot, &c. also maintained a change of curvature, but only in the 

 crystalline. This last is the opinion of Young ; and two me- 

 moirs which he successively presented to the Royal Society of 

 London, contain its complete development. 



In the former, the question is investigated principally in an 

 anatomical point of view. Here Young demonstrated, by means 

 of direct and very delicate instruments, that the crystalline is 

 endowed with a fibrous or muscular constitution which is admi- 

 rably adapted to all varieties of change of form ; and this disco- 

 very overturned the only solid objection which previously had 

 existed against the hypothesis of Sauvages, &c. Scarcely was 

 this discovery pubHshed, when Hunter claimed it as his own ; 

 and by so doing, the celebrated anatomist promoted the interest 

 of the young physician, because his memoir had never been pub- 

 lished, and had been communicated to no one. But this part 

 of the discussion speedily lost its interest ; for it was soon de- 

 monstrated that Leeuwenhoek, by means of his poweful mi- 

 croscopes, had previously observed and represented, in all their 

 ramifications, the muscular fibres of the crystalline of a fish. To 



