472 THE SENSORIAL FUNCTIONS. 



justment exactly corrects the deficiency of re- 

 fraction, which always takes place in the central 

 portions of a lens composed of a material 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 different 

 distances, and which may be called the Aherra- 

 tions of Parallax. When the distance of the 

 object is very great, the rays proceeding from 

 each point arrive at the eye with so little 

 divergence, that each pencil may be considered 

 as composed of rays which are parallel to each 

 other ; the actual deviation from parallelism 

 being quite insensible. But if the same object 

 be brought nearer to the eye, the divergence of 

 the rays becomes more perceptible ; and the 

 effect of the same degree of refraction is to 

 collect them into a focus more remote than 

 before. I For every distance of the object there 



* Sir David Brewster has ascertained that the variations of 

 density producing the doubly refracting structure, in the crys- 

 talline lens of fishes, are related, not to the centre of the lens, 

 but to the diameter which forms the axis of vision : an arrange- 

 ment peculiarly adapted for correcting the spherical aberrations. 

 Philos, Trans, for 1816, p. 317. 



t This is illustrated by Fig. 412, 413, and 414 ; the first of 

 which shows the rapid convergence of rays proceeding from a 

 very distant object, and which may be considered as parallel. 



