250 Memoir of Dr. Thomas Young. 



Young examines first the hypothesis of a variation in the 

 curvature of the cornea, by microscopical observations, 

 which would have rendered the smallest variations appre- 

 ciable ; or we may say more distinctly, he places the eye in 

 certain conditions where the changes in the curvature 

 should be without any effect, he plunges it into water and 

 proves, even then, that the faculty of seeing at different 

 distances, remains in it complete. 



The second of the three possible suppositions, that of an 

 alteration in the dimensions of the organ, is then destroyed 

 by a number of objections and experiments which it would 

 be difficult to overturn. 



The problem appeared definitely settled. For it is obvious, 

 that if, of three possible solutions, two are removed, the 

 third necessarily follows, that the radius of curvature of 

 the cornea and the longitudinal diameter of the eye being 

 unalterable, the form of the crystalline lens must vary. 

 Young, however, did not stop here, he proved directly, by 

 the subtile phenomena of the change inform of the images, 

 that the lens really changes its curvature ; he invented, or 

 at least, improved an instrument susceptible of being em- 

 ployed, even by persons of little intelligence, and little ac- 

 customed to delicate experiments ; and provided with this 

 new method of investigation, he renders it certain that all 

 those who want the crystalline lens, as in consequence of 

 the operation for cataract, do not enjoy the faculty of seeing 

 precisely at different distances. 



It is truly matter of astonishment, that this admirable 

 theory of vision, this fine wove network, where reasoning 

 and the most ingenious experiments afford mutual support, 

 does not occupy the important place in science which it 

 ought. But to explain this anomaly, we must necessarily 

 recur to a sort of fatality. Young, had he been, as he 

 often said with regret, a new Cassander, incessantly pro- 

 claiming important truths, his ungrateful contemporaries 

 would have refused to receive them. We should be speak- 

 ing, it appears to me, less poetically, and with more truth, 

 in remarking, that the discoveries of Young are unknown 

 to the most of those who should be best able to appreciate 

 them. Physiologists do not read his excellent memoir, for 

 he takes for granted the acquisition of more mathematical 

 knowledge than is usually cultivated by the faculty. 





