VISION. 4*21 



rects the deficiency of refraction, which always 

 takes place in the central portions of a lens com- 

 posed 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 Aberrations 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. This is illus- 

 trated by Fig. 412, 413, and 414 ;t the first of which 

 shows the rapid convergence of rays proceeding 

 from a very distant object, and which may be con- 

 sidered as parallel. The second shows that di- 

 vergent rays unite at a more distant focus; and 

 the third, that the focus is more distant the greater 

 the divergence. For every distance of the object 

 there is a corresponding focal distance ; and when 

 the eye is in a state adapted for distinct vision at 



* Sir David Brewster has ascertained that the variations of den- 

 sity producing the doubly refracting structure, in the crystalline lens 

 ,of fishes, are related, not to the centre of the lens, but to the diameter 

 which forms the axis of vision ; an arrangement peculiarly adapted 

 for correcting tiie spherical aberrations. Philos. Trans, for 18 16, 

 p. 317. t Pages 409 and 410. 



