SPHERICAL ABERRATION. 649 



around the image, owing to the unequal refraction which 

 the elementary colours undergo. In the optical instru- 

 ments 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 recom- 

 bining 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 un- 

 equal 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 received 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 or less distinct appearance of colours 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, indistictness 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 dis- 

 tances 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 distance 

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

 retina. 



This power of adaptation of the eye to vision at different 

 distances has received the most varied explanations. It is 



