170 OF SIGHT. 



275. The focus of the refracted rays must fall exactly 

 on the retina, so that the point of vision be neither pro- 

 duced beyond it nor shortened enough to strike on the 

 vitreous body. 



The latter defect exists in short-sighted persons, from 

 the too great convexity of the cornea or gibbosity of 

 the lens. 



The former is the defect of long-sighted persons, 

 in whom there is the opposite conformation of parts. 



276. Since a perfect and sound eye beholds near and 

 remote objects with equal distinctness, it must of neces- 

 sity be supplied with appropriate powers of accommo- 

 dation.* That these internal changes of the eye are 

 chiefly accomplished by the pressure of the straight 

 muscles of the ball, I am clearly convinced, from this 

 among other arguments, that in the Greenland whale 

 an amphibious animal which must see in media of 

 different densities, nature has most accurately provided 

 for it, in the remarkable structure and obsequious 

 flexibility of the sclerotica.f (C) 



277. During the waking state, the eyes are perpe- 

 tually, although insensibly, agitated, and directed to- 

 wards the axes of objects, by these muscles. 



For, although the whole of the retina is sensible, it 

 is not all equally calculated to receive the images 

 of objects. 



In the first place, the true axis of the human J eye, 



* H. W. Math. Olbers, De Oculi Mutationibus Internis. Getting. 1780. 4to. 



Ever. Home, Philos, Trans. 1795. p. 1, 



f Comment. Societ. Scient. Gottinsfen. 1 ;. T. vii. p. 62. fig. ii. f. g. h. 



J I say the human eye ; for in some animals now before me, the seal and 

 porcupine, for instance, the true and imaginary axis are the same, the optic 

 nerve lying exactly opposite the. centre of the cornea and pupil. 



