VISION. 333 



The accurate converp;cnce of all the rays of light, which 

 enter through the pupil, to their respective foci on the reti- 

 na, is necessary for the perfection of the images there formed; 

 but, for the complete attainment of this end, various nice ad- 

 justments are still requisite. 



In the first place, the Aberration of Sphericity * which 

 is a consequence of the geometrical law of refraction, intro- 

 duces a degree of confusion in the image; which is scarcely 

 perceptible, indeed, on a small scale, but which becomes 

 sensible in instruments of much power; being one of the 

 greatest difficulties which the optician has to overcome in 

 the construction of the telescope and the microscope. Na- 

 ture, in framing the human eye, has solved this difficulty by 

 the simplest, yet most effectual means, and in a manner quite 

 inimitable by human art. She has, in the first place, given 

 to the surfaces of the crystalline lens, instead of the spheri- 

 cal form, curvatures more or less hyperbolical or elliptical; 

 and has, in the next place, constructed the lens of an infinite 

 number of concentric layers, which increase in their densi- 

 ty, as they succeed one another from the surface to the cen- 

 tre. The refracting power, being proportional to the 

 density, is thus greatest at the centre, and diminishes as we 

 recede from that centre. This admirable adjustment exact- 

 ly corrects the deficiency of refraction, which always takes 

 place in the central portions of a lens composed of a mate- 

 rial of uniform density, as compared with the refraction of 

 the parts more remote from the centre.! 



The second adjustment for perfect vision has reference to 

 the variations in the distance of the focus which take place 

 according as the rays arrive at the eye from objects at diffe- 

 rent distances, and which may be called the Merrations of 



* See Fig. 411, and the note referring to it, p. 324. 



■j- Sir David Brewster has ascertained that the variations of density pro- 

 ducing- the doubly refracting structure, in the crystalline lens of fishes, arc 

 related, not to the centre of the lens, but to the diameter which forms the 

 axis of vision: an arrangement peculiarly adapted for correcting the splicri- 

 cal aberrations. Philos. Trans, for 1810, p. 317. 



