424 THE SENSORIAL FUNCTIONS. 



light are refracted, under similar circumstances, in 

 dift'erent degrees. Hence the more refrangible 

 rays, namely the violet and the blue, are brought 

 to a nearer focus, than those which are less re- 

 frangible, namely the orange and the red rays ; 

 and this want of coincidence in the points of con- 

 vergence of these different rays, (all of which enter 

 into the composition of white light), necessarily 

 impairs the distinctness of all the images produced 

 by refraction ; shading off their outlines with va- 

 rious colours, even when the object itself is colour- 

 less. This defect, which is incident to the power 

 of a simple lens, and which is termed the Chro- 

 matic Aberration, is remedied almost perfectly in 

 the eye, by the nice adjustment of the powers of the 

 different refracting media, which the rays of light 

 have to traverse before they arrive at the retina, 

 producing what is called an achromatic combi- 

 nation;* and it is found that the eye, though not an 

 absolutely achromatic instrument, as was asserted 

 by Euler,t is yet sufficiently so for all the ordinary 

 practical purposes of life. 



The object, then, of the whole apparatus ap- 

 pended to the optic nerve, is to form inverted 

 images of external objects on the retina, which, as 

 we have seen, is the expanded extremity of that 

 nerve. That this effect is actually produced, may 

 be easily shown by direct observation : for if the 

 sclerotic and choroid coats be . carefully dissected 



* For the exposition of the principles on winch these achromatic 

 combinations of lenses correct this source of aberration, I must refer 

 to works which treat professedly on Optics. 



t For the rectification of this enor we are indebted to Dr. 

 Youn^. 



