CHROMATIC ABERRATION 735 



part of the lens but its center, which corresponds to the pupil. The iris is 

 pigmented to prevent the passage of rays of light through its substance. 

 The image of an object will be most clearly denned and distinct when the 

 pupil is small, if the light is abundant; so that, while a sufficient number 

 of rays are admitted, the narrowness of the pupil may prevent the pro- 

 duction of indistinctness of the image by spherical aberration. But even 

 the image formed by the rays passing through the circumference of the 

 lens, when the pupil is much dilated, as in the dark or in a feeble light, 

 may be well denned. Some types of optical apparatus are corrected for 

 this defect by a central instead of marginal diaphragm. 



Distinctness of vision is further secured by the pigment of the outer sur- 

 face of the retina and of the posterior surface of the iris and the ciliary proc- 

 esses. This absorbs any rays of light that may be reflected within the eye 

 and prevents their being thrown again upon the retina so as to interfere 

 with the images formed there. The pigment of the retina is especially im- 

 portant in this respect; for with the exception of its outer layer the retina is 

 very transparent; and if the surface behind it were not of a dark color, but 

 capable of reflecting the light, the luminous rays which had already acted 

 on the retina would be reflected again and would fall upon other parts of 

 the same membrane, producing indistinctness of the images. 



Chromatic Aberration. In the passage of light through the periphery 

 of an ordinary convex lens decomposition of each ray into its elementary 

 colors commonly ensues, and a colored margin appears around the image, 

 owing to the unequal refraction which the elementary colors undergo. 

 This is termed chromatic aberration. It is corrected by the use of lenses 

 constructed of alternate layers of glass of different refractive indices so 

 ground that they produce chromatic dispersion in opposite directions and 

 thus mutually correct any chromatic aberration which may have resulted. 

 The human eye has considerable chromatic aberration, as may readily be 

 demonstrated, Experiment 13, page 721. 



An ordinary ray of white light in passing through a prism has its con- 

 stituent rays refracted in unequal degrees, and therefore appears as colored 

 bands fading off into each other, known as the spectrum. The colors of 

 the spectrum are arranged as follows; red, orange, yellow, green, blue, 

 indigo, violet; of these the red ray is the least, and the violet the most 

 refracted. Hence, as Helmholtz has shown, the rays from a point of white 

 light cannot be accurately focused on the retina, for if we focus for the 

 red rays, the violet are out of focus, and vice versa; such objects, if not ex- 

 actly focused, are often seen surrounded by a pale yellowish or bluish 

 fringe. 



For similar reasons a red surface looks nearer than a blue one at an 

 equal distance, because, the red rays being less refrangible, a stronger 

 effort of accommodation is necessary to focus them, and the eye is ad- 



