VISION. 471 



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 * 

 which is a consequence of the geometrical law 

 of refraction, introduces 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. 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 in- 

 crease in their density, as they succeed one ano- 

 ther from the surface to the centre. The refract- 

 ing power, being proportional to the density, is 

 thus greatest at the centre, and diminishes as it 

 recedes from that centre. This admirable ad- 



* See Fig. 411, and the note referring to it, p. 457. 



