244 ^p 



vexity of the cornea and the crystalline, the greater density of the hu- 

 mours, or the excessive depth of the ball, the rays of light, too soon re- 

 united, diverge anew, fall scattering on the retina, and yield only a con- 

 fused sensation. In this defect of sight, called .myojiia, the eye distin- 

 guishes only very near objects, giving out rays of such extreme diver- 

 gence as to require a very powerful refractor. In firesbytia^ on the 'other 

 hand the cornea too much flattened, the crystalline little convex, or set 

 too deeply, the humonrs too scanty, are the cause that the rays are not 

 yet collected when they fall upon the retina, so that none but very distant 

 objects are distinctly seen, because the scarcely divergent rays they^give 

 out, have no need of much refraction. 



Myopia is sometimes the effect of the. habit which some children get 

 of looking very close at objects which catch their attention. The pupil 

 then becomes accustomed to great constriction, and dilates afterwards 

 with difficulty. It is obvious, that to correct this vicious disposition, 

 you must show the child distant objects which will strongly engage his 

 curiosity, and keep him at some distance from every thing he looks at. 



The sensibility of the retina, on some occasions, rises to such excess, 

 that the eye can scarcely bear the impression of the faintest light. Nyc- 

 taclofies, such is the name given to those affected with this disorder, dis- 

 tinguish objects amidst the deepest darkness 5 a few rays are sufficient 

 to impress their organ. 



It is related that an English gentleman, shut up in a dark dungeon, 

 came gradually to distinguish all it contained: when he returned to the 

 light of day, of which he had in some sort lost the habit, he could not 

 endure its splendour, the edges of the pupil, before extremely diluted, be- 

 came contracted to such a degree as entirely to efface the opening. 



When on the other hand, the retina has little sensibility, strong day- 

 light is requisite to sight. This injury of vision, known by the name of 

 hemeralofiia't, may be considered as the first step of total paralysis of the 4 

 oplic nerve, or gutta serena. It may arise from any thing that can impair 

 the sensibility of the retina. Saint-Yves relates, in his work on diseases 

 of the eyes, many cases of hemeralopia. The subjects were chiefly work- 

 men, employed at the Hotel des Monnoies, in melting the metals. The 

 inhabitants of the northern regions, where the earth is covered with snow 

 great part of the year, become at an early age hemeralopes. Both con- 

 tract this weakness, from their eyes being habitually fatigued by the 

 splendour of too strong a light. 



Finally, in order to the completion of the mechanism of vision, it is re- 

 quisite that all parts of the eye be under certain conditions, the want of 

 which is more or less troublesome. It is especially necessary, that the 

 membranes and the humours which the rays of light are to pass through, 

 should be perfectly transparent. Thus specks of the cornea, the closing 

 of the pupil by the preservation of the membrane which stops that open- 



* I give to the words nyctalopia and hemeralopia the same meaning- as all other writers 

 down to Scarpa, who has published the latest Treatise on diseases of the eyes. This 

 acceptation is, however, a grammatical error, since the two terms, nyctalopia, in its Greek 

 roots, signifies an affection* which takes away sight during- the night, and hemeralopia 

 one in which it is lost during the day. It is "accordingly in this sense that they are used 

 by the father of physic. I owe this remark to Dr. Houssille Chamseru, who has care- 

 fully verified the text of Hippocrates in the MS S of the Imperial Librarv Author's 

 Note. 



