46 INNERVATION. [CHAP. XVII. 



To apply tliis to the eye. If a luminous object, as tlie flame of 

 a candle, be placed eight or ten inches in front of the organ, some 

 rays fall on the sclerotic and are reflected ; the more central ones 

 fall on the cornea : some are reflected, and others pass through it, 

 are slightly converged by it, and enter the aqueous humor, which, 

 being probably of the same refracting power, does not alter their 

 course. Passing onwards, some meet the iris and are absorbed or 

 reflected by it, whilst others advance through the pupil. Thus rays, 

 falling on a large extent of the cornea, are converged so as to fall 

 on the lens. By the convexity of the surface of the lens, as well as 

 by the greater density of that body towards its centre, this conver- 

 gence is much increased. Lastly, by their passage into the rarer 

 medium of the vitreous humor the rays are further converged by the 

 refraction of each ray from the perpendicular to the point of inci- 

 dence, and the several pencils which they form are brought to as 

 many foci in the retina. And still further, the rays from the oppo- 

 site points of the luminous object, by reason of the change of direc- 

 tion which they undergo through these successive refractions, cross 

 one another, (the angle of crossing being called the visual angle,) 

 and thus the image of the flame on the retina appears inverted. 



This inversion of the image may be exhibited by a model, repre- 

 senting the transparent media of the eye, with a retina of ground 

 glass; or it may be shown on a recent eye by simply removing the 

 opaque coats behind the retina, or in the eye of a white rabbit, after 

 removing the muscles and areolar tissue around it. 



When the retina corresponds, or nearly so, to the points of con- 

 vergence of the several pencils of light, distinct vision of the object 

 is obtained; and the distance for distinct vision is ordinarily about 

 ten inches. If that distance be increased or diminished (no change 

 being produced in the eye), vision is indistinct; for when the object 

 is removed to a greater distance from the eye, it is obvious that the 

 focus will be moved forwards, or will fall short of the retina; and 

 when the object is approximated, the focus will be moved backwards 

 or beyond the retina : in both which cases the same point of the re- 

 tina will receive rays from several points of the object. Hence it 

 is, that when the eye is adapted to distinct vision at a distance of 

 ten inches, we cannot distinctly see objects at a greater or less dis- 

 tance. From the cause of this, which has been just alluded to, 

 however, it is evident that, provided the rays unite very nearly on 

 the retina, vision, especially of large objects, may prove sufficiently 

 distinct, although not perfectly clear. Hence the distinction of 

 Jurin between distinct and perfect vision is worthy of being borne 



