526 VISION 



distance between the two focal lines was only 0.03 mm. and the degree of 

 astigmatism only 0.1 diopter. These figures explain why, as has long been 

 known, the sharpness of vision commonly suffers no reduction from this 

 kind of astigmatism. 



E. CHROMATIC ABERRATION IN THE EYE 



The refractive index of solid and liquid media is different for rays of 

 different wave length e. g., that of water for red (spectrum line C) is 1.331705 

 and for violet (spectrum line Gr) 1.341285. For a long time it was supposed 

 to be impossible to prevent this dispersion of light into its colors in any optical 

 system. Later, however, it was shown to be possible and instruments have 

 long since been constructed in which no color dispersion at all occurs. 



Our everyday experience teaches us that the chromatic aberration in the 

 eye cannot be very great, for in ordinary life it is almost entirely unnoticeable. 

 But more exact investigation of the subject shows that the achromatism of 

 the eye is by no means perfect. 



Since the refractive indices of the optical media in the eye for the most 

 part do not differ much from that of water, Helmholtz calculated the dispersion 

 for the reduced eye (see page 512), on the assumption that water was the 



C 



FIG. 221. Diagram illustrating the chromatic aberration of an eye. 



refractive substance throughout, and found that the posterior focal distance for 

 red (line C) was 20.574 mm. and for violet (line G) 20.14 mm. Actually the 

 color dispersion in the human eye appears to be somewhat greater (the distance 

 between the focal points of red and violet 0.58-0.62 mm. instead of 0.434 mm.). 

 According to Einthoven, the difference in focal distance between the D and F 

 lines in the schematic eye is 0.272 mm. 



There is, however, a physiological reason as well as a structural one why the 

 color dispersion is not plainly noticeable. When white light enters the eye and 

 the eye adjusts itself for the most strongly effective rays of medium wave length, 

 the latter come together on the retina almost exactly in one point, which is 

 surrounded by a fringe of red and violet rays. But the exciting effect of rays 

 of very great or very small wave length is relatively slight, consequently the 

 section of the fringe zone in comparison with that of the center is negligible. 

 Besides, the center is more strongly illuminated than the fringe zone because 

 lays of all wave lengths strike it. 



The same thing is true of the dispersion circles caused by the spherical 

 aberration when the eye is adjusted to the focal point of the central rays. 



Only one experiment on color dispersion in the eye can be described here. 

 If one holds before an ordinary petroleum flame a screen with a narrow open- 

 ing in it and behind this a cobalt-blue glass which shuts out most of the orange, 

 yellow and green rays, but lets through an abundance of ultra-red, indigo-blue 

 and violet rays, the opening may be seen as a luminous point sending out red 



