510 THE SENSE OP SIGHT. 



commonly ensues, and a colored margin appears around the 

 image, owing to the unequal refraction which the elementary 

 colors undergo. In the optical instruments this, which is 

 termed chromatic aberration, is corrected by the use of two or 

 more lenses, differing in shape and density, the second of which 

 continues or increases the refraction of the rays produced by 

 the first, but by recombining the individual parts of each ray 

 into its original white light, corrects any chromatic aberration 

 which may have resulted from the first. It is probable that 

 the unequal refractive power of the transparent media in front 

 of the retina may be the means by which the eye is enabled to 

 guard against the effect of chromatic aberration. The human 

 eye is achromatic, however, only so long as the image is re- 

 ceived at its focal distance upon the retina, or so long as the 

 eye adapts itself to the different distances of sight. If either 

 of these conditions be interfered with, a more er less distinct 

 appearance of colors is produced. 



2. The distinctness of the image formed upon the retina is 

 mainly dependent on the rays emitted by each luminous point 

 of the object being brought to a perfect focus upon the retina. 

 If this focus occur at a point either in front of, or behind the 

 retina, indistinctness of vision ensues, with the production of a 

 halo. The focal distance, i. e., the distance of the point at which 

 the luminous rays from a lens are collected, besides being 

 regulated by the degree of convexity and density of the lens, 

 varies with the distance of the object from the lens, being 

 greater as this is shorter, and vice versa. Hence, since objects 

 placed at various distances from the eye can, within a certain 

 range, different in different persons, be seen with almost equal 

 distinctness, there must be some provision by which the eye is 

 enabled to adapt itself, so that whatever length the focal dis- 

 tance may be, the focal point may always fall exactly upon 

 the retina. 



This power of adaptation of the eye to vision at different dis- 

 tances has received the most varied explanations. It is ob- 

 vious that the effect might be produced in either of two ways, 

 viz., by altering the convexity or density, and thus the refract- 

 ing power, either of the cornea or lens ; or, by changing the 

 position either of the retina or of the lens, so that whether the 

 object viewed be near or distant, and the focal distance thus 

 increased or diminished, the focal point to which the rays are 

 converged by the lens may always be at the place occupied by 

 the retina. The amount of either of these changes required in 

 even the widest range of vision, is extremely small. For, from 

 the refractive powers of the media of the eye, it has been cal- 

 culated by Olbers, that the difference between the focal dis- 



