THE SENSES. 603 



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 indistinct- 

 ness of the images. 



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

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

 parts 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 

 corrected by the use of two or more lenses, differing in shape and dens- 

 ity, 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 re- 

 fractive 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 chro- 

 matic 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 re- 

 fracted, 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 there- 

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

 band known as the " spectrum " is produced, the colors of which 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, a small white object cannot be accurately fo- 

 cussed on the retina, for if we focus for the red rays, the violet are out 

 of focus, and vice versa : such objects, if not exactly focussed, 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- 

 justed as if for a nearer object, and therefore the red surface appears 

 nearer. 



From the insufficient adjustment of the image of a small white ob- 

 ject, it appears surrounded by a sort of halo or fringe. This phenome- 

 non is termed Irradiation. It is from this reason that a white square 

 on a black ground appears larger than a black square of the same size on 

 ground. 



