ON VISION. 351 



humour, which fills its concavity, and distends it. This humour is par- 

 tially divided by the uvea or iris, which is of different colours in different 

 persons, having a perforation in its centre, called the pupil. Immediately 

 behind the uvea, and closely connected to its base, are the ciliary processes, 

 the summits of which hang like a short fringe, before the crystalline lens, 

 a substance much more refractive than the aqueous humour, and increas- 

 ing in density towards its centre. The remaining cavity is filled by an 

 aqueous fluid, lodged in a cellular texture of extremely fine membrane, 

 and called the vitreous humour. The retina lines the whole posterior 

 part a of this cavity ; it is semitransparent, and is supported by the choroid 

 or chorioid coat, a very opaque black or brown membrane, continued from 

 the uvea and ciliary processes ; but immediately where the retina is con- 

 nected witUPbhe optic nerve, the choroid is necessarily perforated ; and at 

 this part a small portion of the retina is nearly insensible. The whole is 

 surrounded by an opaque continuation of the cornea, called the sclerotica.^ 



The rays of light which have entered the cornea and passed through the 

 pupil, being rendered still more convergent by the crystalline lens, are 

 collected into foci on the retina, and form there an image, which, according 

 to the common laws of refraction, is inverted, since the central rays of each 

 pencil cross each other a little behind the pupil ; and the image may easily 

 be seen in a dead eye, by laying bare the posterior surface of the retina. 

 (Plate XXX. Fig. 437.) 



By means of this arrangement of the various refracting substances, 

 many peculiar advantages are procured. The surface of the cornea only, 

 if it had been more convex, could not have collected the lateral rays of a 

 direct pencil to a perfect focus, without a different curvature near its 

 edges ; and then the oblique pencils would have been subjected to greater 

 aberration, nor could they have been made to converge to any focus on the 

 retina. A second refraction performs both these offices much more com- 

 pletely, and has also the advantage of admitting a greater quantity of 

 light. If also the surfaces of the crystalline lens thus interposed, had been 

 abrupt, there would have been a reflection at each, and an apparent 

 haziness would have interfered with the distinct view of every luminous 

 object; but this inconvenience is avoided by the gradual increase of 

 density in approaching the centre, which also makes the crystalline equiva- 

 lent to a much more refractive substance of equal magnitude ; while, at 

 the same time, the smaller density of the lateral parts prevents the usual 

 aberration of spherical surfaces, occasioned by the too great refraction of 

 the lateral rays of direct pencils, and causes also the focus of each oblique 

 pencil to fall either accurately or very nearly on the concave surface of 

 the retina, throughout its extent. 



Opticians have often puzzled themselves, without the least necessity, in 

 order to account for our seeing objects in their natural erect position, while 

 the image on the retina is in reality inverted : but surely the situation of a 

 focal point at the upper part of the eye could be no reason for supposing the 

 object corresponding to it to be actually elevated. We call that the lower 

 end of an object which* is next to the ground ; and the image of the trunk 

 of a tree ,being in contact with the image of the ground on the retina, we 



