574 THE NERVOUS SYSTEM. 



i. e., undergoes dispersion and the most refrangible or violet 

 rays will be brought to a focus nearer the lens (Fig. 352) than the 

 least refrangible or red rays, and between will be the various 

 intermediate colors. When these rays fall upon a screen there 

 will be produced a series of circles of the different colors. In 

 Fig. 352 it will be seen that if the screen was placed at x, the 

 outer color would be red, and the inner violet ; while if it was 



FIG. 352. Chromatic aberra- FIG. 353. Achromatic combination 



tion (Carhart and Chute). of lenses (Carhart and Chute). 



placed at y, the colors would be reversed. This defect in lenses 

 is chromatic aberration, and is overcome by combining a biconvex 

 lens of crown glass with a planoconcave lens of flint glass 

 (Fig. 353). Inasmuch as images formed by rays which have 

 passed through such a combination are not fringed with color, the 

 combination is achromatic. 



It is a rather curious fact that the eye was at one time sup- 

 posed to be free from this defect, and its absence was explained 

 by the fact that the different media through which light passes 

 to reach the retina differ so in their refracting power as to over- 

 come dispersion ; and it is said that it was this which led to the 

 combination just described of the crown and flint glass to make 

 the achromatic lens. The media of the eye, however, do not 

 form an achromatic combination, but the violet rays are actually 



FIG. 354. To show dispersion in the eye, view the figure from a distance too small 

 for accommodation. Approach the eye toward it : the white rings appear bluish, 

 owing to circles of dispersion falling on them. A little closer, and the black rings 

 become white or yellowish-white, being covered by circles of dispersion and diffusion. 



brought to a focus about 0.5 mm. in front of the red. Under 

 ordinary circumstances this produces no confusion ; and yet that 

 this defect is inherent in the human eye may be readily demon- 

 strated. If Fig. 354 is brought very close to the eyes so close 

 that the two crystalline lenses cannot accommodate for it the 

 white rings become bluish on account of circles of dispersion fall- 

 ing on them, and if brought a little closer, the black rings become 

 of a yellowish-white color. This dispersion also explains irradia- 

 tion. 



