EULOGY OX THOMAS YOUNG. 117 



senses, and which have been stigmatized under the porticoes of the 

 Academy by the contemptuous epithet of sinqyle opinion. Such is 

 always human weakness that, after having IbUowed with a rare success 

 the principal deviations whicli light uudergoes in passing through the 

 cornea and the crystalline, ]Maurolycus and Porta, when very near attain- 

 ing their object, stopped short, as if before an insurmountable difficulty, 

 when it was objected to their theory that objects ought to appear in an 

 inverted position if the images formed in the eye are themselves in- 

 verted. The adventurous spirit of Kepler, on the contrary, did not re- 

 main embarrassed. It was from psychology that the attack originated ; 

 it was equally from psychology, clear, precise, and mathematical, that he 

 overthrew the objection. Under the powerful hand of this great man 

 the eye became, definitively, the simple optical apparatus known by the 

 name of the camera-ohscura ; the retina is the ground of the picture, 

 the crystalline replaces the glass lens.* 



This assimilation, generally adoi)ted since Kepler's time, remains open 

 only to one difficulty ; the, camera-obscura^ like an ordinary telescope, re- 

 quires to be brought to a proper focus, according to the distance of ob- 

 jects. When objects are near, it is indispensable to increase the distance 

 of the picture from the lens ; a contrary movement becomes necessary 

 as they become more distant. To preserve to the images all the dis- 

 tinctness which is desirable, without changing the position of the sur- 

 face which receives them, is therefore impossible; at least, always sup- 

 posing the curvature of the lens to remain invariable, that it cannot 

 increase when we look at near objects, or diminish for distant objects. 



Among the different modes of obtaining distinct images, nature has 

 assuredly made a choice, since man can see with great distinctness at 

 renj (liferent distances. The question thus juit has aftbrded 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. Porter- 

 field andZinn contended that the crystalline lens was movable, and that 

 it could place itself nearer to or further from the retina, as might be 

 needed. Juriu and ^Nlusschenbrceck believed in a change in the curva- 

 ture 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 suc- 

 cessively submitted to the Koyal Society of London, include the com- 

 plete development of his views. 



In the first ot 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 en- 

 dowed 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 subject, 



* The author seems to have left this ilhistration iiicoiypletc. Kepler's suggestion of 

 the identity of the ej-e with the camera-obscnra, after all, does not touch the ditiic-ulty 

 of the hiversion of the image. Nor has it lieeu considered as completely cleared np even 

 till much later times. The solution which, it is believed, is now most generally as- 

 sented to is this : It is a law of our constitution, dependent on some physiological 

 principle unknown, that we refer impressions on the retina to objects existing, or be- 

 lieved to exist, in the rectilineardirection /com ic//ic/) the imin'cssion comes to the retina. 

 Consequently, as rays cross at the pupil, falling on the Hjy^cc part of the retina a ray 

 suggests an object lying helow, or an inverted image suggests an erect object. 



