240 SENSE OF SIGHT. 



they might be rendered achromatic. Experience has shown the accu- 

 racy of this opinion (Fig. 83). Others have believed, that the effect is 

 produced by certain of the humours as the aqueous and vitreous 

 which they have considered capable of correcting the dispersion pro- 

 duced by the cornea and crystalline. Others, again, have placed it in 

 the crystalline, the layers of which being of different dispersive powers 

 may correct each other. Lastly; some have denied altogether the 

 necessity for the eye's being achromatic; asserting, that the depth of 

 the organ is so inconsiderable, that the dispersion of the rays, by the 

 time they reach the retina, ought to be inappreciable. This was the 

 opinion of M. D'Alembert. Dr. Maskelyne 1 calculated the amount of 

 the aberration, that must necessarily take place in the eye, and con- 

 cluded that it would be fourteen or fifteen times less than in a common 

 refracting telescope; and therefore imperceptible. Uncertainty still 

 rests on this subject; and it cannot be removed until the dispersive 

 and refractive powers of the transparent parts of the organ as well as 

 their exact curvatures shall have been mathematically determined. It 

 has been already shown, that the data we possess on this subject from 

 different observers are sufficiently imprecise. 



Our knowledge, then, is restricted to the fact, that the eye is per- 

 fectly achromatic; and that, in this respect, it exceeds any instrument 

 of human construction. The views of Euler are the most probable; 

 and the effect doubtless is much aided by the iris or diaphragm, which 

 prevents the rays from falling upon the margins of the lens, where, by 

 the surfaces meeting at an angle, the aberration must necessarily be 

 greatest. 



Of the coats of the eye, the sclerotic gives form to, and protects the 

 organ. 



The choroid is chiefly useful by the black pigment, which lines and 

 penetrates it. It will be seen that some individuals, on insufficient 

 grounds, have esteemed it the seat of vision. Leaving this question for 

 the moment, and granting, as we shall endeavour to establish, that the 

 impression is received upon the expansion of the optic nerve the 

 retina, the use of the choroid would seem to be, in ordinary circum- 

 stances, to afford surface for the pigmentum nigrum, whose function it 

 is to absorb the rays after they have passed through the retina; and 

 thus to obviate the confusion, that would arise from varied reflections, 

 we're the choroid devoid of such dark covering. In albinos or white 

 animals, in which the pigment is wanting, this inconvenience is really 

 experienced; so that they become nyctalopes, or, at least, see but im- 

 perfectly during the day. In the night, or when the light is feeble, 

 their vision is unimpaired; hence the albinos of our species have been 

 called by the Germans and Dutch, Kakerlaken or cockroaches. Sir 

 Everard Home 2 is of opinion, that the pigmentum nigrum is provided 

 as a defence against strong light; and that, hence, it is lightest in those 

 countries least exposed to the scorching effects of the sun. In con- 



1 Philosoph. Transactions for 1789, Ixix. 256. 



2 Lectures on Comparative Anatomy, iii. 220, Lond., 1823. 



