S P H E R I C A L A B E R II A T I () N. 509 



1. Since the retina is concave, and from its centre towards 

 its margins gradually approaches the lens, it follows that the 

 images of objects situated at the sides cannot be so distinct as 

 those of objects nearer to the middle of the field of vision, and 

 of which the images are formed at a distance beyond the lens 

 exactly corresponding to the situation of the retina. More- 

 over, the rays of a cone of light from an object situated at the 

 side of the field of vision do not meet all in the same point, 

 owing to their unequal refraction ; for the refraction of the 

 rays which pass through the circumference of a lens is greater 

 than that of those traversing its central portion. The concur- 

 rence of these two circumstances would cause indistinctness of 

 vision, unless corrected by some contrivance. Such correction 

 is effected, in both cases, by the iris, which forms a kind of 

 annular diaphragm to cover the circumference of the lens, and 

 to prevent the rays from passing through any part of the lens 

 but its centre, which corresponds to the pupil. 



The image of an object will be most defined and distinct 

 when the pupil is narrow, the object at the proper distance for 

 vision, and the light abundant; so that, while a sufficient 

 number of rays are admitted, the narrowness of the pupil may 

 prevent the production of indistinctness of the image by this 

 spherical aberration or unequal refraction just mentioned. 

 But even the image formed by the rays passing through the 

 circumference of the lens, when the pupil is much dilated, as 

 in the dark, or in a feeble light, may, under certain circum- 

 stances, be well defined ; the image formed by the central rays 

 being then indistinct or invisible, in consequence of the retina 

 not receiving these rays where they are concentrated to a 

 focus. - 



Distinctness of vision, is further secured by the inner sur- 

 face of the choroid, immediately external to the retina itself, 

 as well as the posterior surface of the iris and the ciliary pro- 

 cesses, being coated with black pigment, which absorbs any 

 rays of light that may be reflected within the eye, and pre- 

 vents their being thrown again upon the retina so as to inter- 

 fere with the images there formed. The pigment of the cho- 

 roid is especially important in this respect ; for the retina is 

 very transparent, and if the surface behind it were not of a 

 dark color, but capable of reflecting the light, the luminous 

 rays which had already acted on the retina would be reflected 

 again through it, and would fall upon other parts of the same 

 membrane, producing both dazzling from excessive light, and 

 indistinctness of the images. 



In the passage of light through an ordinary convex lens, 

 decomposition of each ray into its elementary colored parts 



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