EXPERIMENT OF MARIOTTE. 253 



course of a few days, were entirely opaque. Eight days after the divi- 

 sion of the nerve, the cornea separated from the sclerotica ; and the 

 portions of the humours that remained fluid escaped at the opening. 

 The organ diminished in size, and ultimately became a kind of tubercle, 

 filled with a substance of a caseous appearance. M. Magendie properly 

 concludes from these experiments, that the nutrition of the eye is under 

 the influence of the fifth pair ; and he conceives, that the opacity of 

 the cornea was directly owing to the section of this nerve, and not to 

 a cessation of the lachrymal secretion, or to the prolonged contact of 

 air, caused by the paralysis of the eyelids; inasmuch as when only the 

 branches of the nerve proceeding to the eyelids were divided, or when 

 the lachrymal gland was taken away, the opacity did not supervene. 



5. PHENOMENA OF VISION. 



It has been more than once remarked, that the retina the expansion 

 of the optic nerve is the part of the eye which receives the impressions 

 of luminous rays, whence they are conveyed by that nerve to the brain. 

 Yet this has been contested. 



The Abbe Mariotte 1 discovered the singular fact, that when a ray of 

 light falls, as he conceived, upon the centre of the optic nerve it excites 

 no sensation. " Having often observed," he remarks, "on dissections 

 of men as well as of brutes, that the optic nerve does never answer 

 just to the middle of the bottom of the eye ; that is, to the place where 

 the picture of the object we look directly upon is made ; and that in 

 man it is somewhat higher, and on the side towards the nose; to make 

 therefore the rays of an object to fall upon the optic nerve of my eye, 

 and to find the consequence thereof, I made this experiment. I fast- 

 ened on an obscure wall, about the height of my eye, a small round 

 paper, to serve me for a fixed point of vision. I fastened such another 

 on the side thereof- towards my right hand, at the distance of about 

 two feet, but somewhat lower than the first, to the end that I might 

 strike the optic nerve of my right eye, while I kept my left shut. Then 

 I placed myself over against the first paper, and drew back by little and 

 little, keeping my right eye fixed and very steady on the same, and being 

 about ten feet distant, the second paper totally disappeared." 



The experiment of Mariotte can be readily repeated on the marginal 

 representations of the fleur- 

 de-lis and arrow. If we close Fi g- H5. 

 the left eye, and direct the axis 1 A 



Of the right eye Steadily to- Experiment of Mariotte. 



wards the arrow, when the 



page is held at the distance of about ten inches from the eye, the fleur- 

 de-lis vanishes. The distance of the object which disappears from the 

 eye must be about five times as great as its distance from the other 

 object. In this case the fleur-de-lis and arrow are two inches asunder. 

 It is obvious, from what has been said, regarding the axis of the orbits, 

 and the part of the eyeball at which the optic nerve enters that rays 



1 Philos. Transact , iii. 668, and Memoir de 1'Academie Royale des Sciences, torn. i. pp. 68, 

 and 102. 



