THE SENSES. 721 



pigment, to prevent the passage of rays of light through its substance. 

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

 pupil is narrow, the object at the proper distance for vision, and the 

 light abundant; so that, while a sufficient number of rays are admitted, 

 the narrowness of the pupil may prevent the production 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, under certain 

 circumstances, be well defined. 



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

 surface of the retina, the posterior surface of the iris and the ciliary 

 processes, which 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 there formed. The pigment of the retina is 

 especially important 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 through 

 it, and would fall upon other parts of the same membrane, producing 

 both dazzling from excessive light, and indistinctness of the images. 



5. Chromatic Aberration. In the passage of light through an ordi- 

 nary convex lens, decomposition of each ray into its elementary colored 

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

 owing to the unequal refraction which the elementary colors undergo. 

 In optical instruments this, which is termed chromatic aberration, is con- 

 nected 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 tihe first. It is probable that the unequal refrac- 

 tive 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 colors is produced. 



An ordinary ray of white light in passing through a prism, is refract- 

 ed, i.e., bent out of its course, but the different colored rays which go 

 to make up white light are refracted in different degrees, and therefore 

 appear as colored bands fading off into each other: thus a colored ban$ 

 known as the "spectrum" is produced, the colors of which are arrange^ 

 as follows red, orange, yellow, green, blue, indigo, violet; of these 

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