194 ADAPTATIONS TO DIURNAL ACTIVITY 



out of 'white' light. The shorter waves are bent most, longer waves pro- 

 gressively less. As Figure 29c (p. 82) shows, this results in a series of 

 focal points beyond a lens, the violet focus being nearest and the red 

 focus farthest away. The distance occupied by these foci is called the 

 linear chromatic aberration, and in the human eye it is considerably 

 more than the whole thickness of the retina. In the refractionist's lan- 

 guage, the aberration amounts to about two diopters. The 'normal' or 



550 



Wavelength (mp) 





.401- m 

 t < 



Q 



Fig. 83 — Graph showing how yellow filters combat chromatic aberration. 



(Curve of transmission spertrum smoothed from data of Ludvigh and McCarthy on absorp- 

 tion in the lens, cornea, and humors of the human eye, integrated with data of M. Sachs 

 on absorption in the most completely studied macula among his nine examples; dispersion 

 curve for the human dioptric media, showing the relative refrangibility of the various wave- 

 lengths, plotted from data of Polack). 



The curves bring out the fact that the short waves, which are most strongly dispersed and 

 which consequently contribute most to chromatic aberration (c/. Fig. 29c, p. 82), are the 

 ones most strongly absorbed (i.e., least well transmitted) by the yellow filters interposed 

 in their path within the eye. 



emmetropic human eye is actually emmetropic only for yellow light, and 

 is, simultaneously, 0.75 diopters hypermetropic for red and 1.25 diopters 

 myopic for violet. Since the dioptric apparatus ordinarily places the 

 yellow focus in the visual-cell layer, we must actually accommodate when 

 diverting our attention from a blue object to a red one at the same 

 actual distance from the eye, and must relax accommodation upon look- 

 ing back at the blue object. This fact is employed by astute artists to 

 heighten the illusion of depth in their paintings. 



