448 • LKCTURE XXXVIII. 



like a short fringe, before the crystalline lens, a substance much more re- 

 fractive than the aqueous humour, and increasing in density towards its 

 centre. The remaining cavity is fdled by an aqueous fluid, lodged in a eel" 

 lular texture of extremely fine membrane, and called the vitreous humour. 

 The retina lines the whole posterior part 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 imme- 

 diately where the retina is connected with the optic nerve, thechoroid 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 cor- 

 nea, 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 col- 

 lected 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 pen- 

 cil 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 completely, 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 equivalent 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 



