474 THE SENSORIAL FUNCTIONS. 



we accurately attend to the accompanying sen- 

 sations. The researches of Dr. Young have ren- 

 dered it probable that some change takes place 

 in the figure of the lens whereby its convexity, 

 and perhaps also its distance from the retina, are 

 increased. He has shown by a very decisive 

 experiment, that any change which may take 

 place in the convexity of the cornea has but 

 little share in the production of the effect ; for 

 the eye retains its power of adaptation when im- 

 mersed in water, in which the form of the cornea 

 can in no respect influence the refraction. 



But the rays of light are of different kinds ; 

 some exciting the sensation of red, others of 

 yellow, and others again of blue ; and these 

 different species of light are refracted, under 

 similar circumstances, in different degrees. 

 Hence the more refrangible rays, that is the 

 violet and the blue, are brought to a nearer 

 focus, than those which are less refrangible, that 

 is the orange and the red rays : and this want 

 of coincidence in the points of convergence 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 various 

 colours, even when the object itself is colourless. 

 This defect, which is incident to the power of a 

 simple lens, and which is termed the Chromatic 

 Aberration, is remedied almost perfectly in the 



