816 THE SENSE OF SIGHT 



is to be expected, since light in passing from a rare into a dense medium 

 suffers a retardation, and this diminution in its velocity affects its 

 component rays differently, i.e., those at the red end, with long wave- 

 lengths, are refracted the least and those at the violet end, with short 

 wave-lengths, the most. Inasmuch as a lens is composed of a series 

 of prisms and prisms split the white light in accordance with the 

 unequal refrangibility of its simple color components a spectrum must 

 result (Fig. 424). Thus, white light, when passed through the edge 

 of a biconvex lens, is dispersed so that its violet rays are brought 

 to a focus (V) in front of its red rays (R), while the foci of its orange, 

 yellow, green, blue and indigo are situated in between these two ex- 

 tremes. This condition which is called chromatic aberration, is also 

 present in the lens of our eye, but cannot seriously interfere with the 

 formation of the image, because the iris does not permit the rays of 



FIG. 424. DIAGRAM ILLUSTRATING CHROMATIC ABERRATION. 



The iria being retracted, the rays of white light traversing the peripheral zones of 

 the lens are split into their spectral components. The violet rays are focalized nearer 

 the lens than the red. 



light to pass through its more poorly refracting peripheral portion. 

 By analogy, it may be concluded that the mydriatic eye receives chro- 

 matically aberrated images, because the edge of its lens is fully exposed 

 to the entering beam of light. In artificial lenses this difficulty is 

 often overcome by combining crown glass with flint glass. Inasmuch 

 as the dispersive power of these glasses is very different, their individual 

 dispersion may thereby be corrected without considerably lessening 

 their total refractive power. This principle has been made use of by 

 Dolland in the construction of the achromatic lenses, achromatism 

 being the term applied to the phenomenon of the refraction of light 

 without decomposition into its components. 



It is a well-known fact that if we gaze at a red and violet light 

 placed at the same distance in front of us, the former appears to be 

 the more prominent and seems nearer to us. Clearly, this is merely 

 an error of judgment, because since the red rays possess a greater 



