420 THE SENSORIAL FUNCTIONS. 



of the eye. The most effective of these refractions 

 is the first; because the difference of density be- 

 tween the air and the cornea, or rather the aqueous 

 humour, is greater than that of any of the humours 

 of the eye compared w^ith one another. 



The accurate convergence of all the rays of light, 

 which enter through the pupil, to their respective 

 foci on the retina, is necessary for the perfection of 

 the images there formed ; but for the complete 

 attainment of this end various nice adjustments are 

 still requisite. 



In the first place, the Aberration of Sphericity, 

 already noticed,* which is a consequence of the 

 geometrical law of refraction, introduces a degree 

 of confusion in the image ; which is scarcely per- 

 ceptible, indeed, on a small scale, but which be- 

 comes 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. Nature, 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 spherical 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 density, as they succeed one another from the 

 surface to the centre. The refracting power, being 

 proportional to the density, is thus greatest at the 

 centre, and diminishes as it recedes from that 

 centre. This admirable adjustment exactly cor- 



* See Fig. 411, p. 410. 



