CHROMATIC ABERRATION OF THE EYE 111 



fact, when white light is used one must take into consideration not only 

 the deviations but also dispersion. 



The convex lens in Fig. Ill— 7 shows how a parallel beam of white 

 light, incident on its left face, is deviated and dispersed. It will be 

 noticed that the violet light is more deviated than the red light; hence 

 the lens has a shorter focal length for violet than for red light. Thus, if a 

 screen is placed at A, we shall get a central white spot surrounded by a 

 violet ring of color surrounded by green and yellow rings with an outside 



Fig. Ill— 7. Chromatic aberration or chromatic difference of focus of a simple 



biconvex lens. 



border of red; if the screen is placed at B, one may observe a white 

 spot surrounded by a red ring of color surrounded by yellow and green 

 rings with an outside border of violet. This difference of focus for 

 rays of different wavelengths is called chromatic difference of focus; it is 

 due to the fact that the index of refraction of the lens increases with a 

 decrease in wavelength. 



If for the sake of simplicity a simplified eye is adopted consisting of 

 one refracting surface enclosing a single medium composed of water, 

 and giving the single refracting surface a radius of 5.13 mm, the differ- 

 ent focal lengths of this eye for red and blue light can be calculated. 

 The indices of refraction of water for red (6536 A) and blue (4308 A) 

 light are 1.331 and 1.342, respectively. The focal lengths of the simpli- 

 fied eye are therefore 20.57 and 20.14 mm. If the eye were unaccom- 

 modated so that the retina lies at the focus of the yellow rays, approx- 

 imately halfway between these foci, the focus of the blue ray would be 

 about 0.22 mm behind the retina. Experiments have shown that, when 

 such a series of foci are formed by the eye, the rays of greatest intensity 

 form the most sharply focused image. For maximum brightness, the 



o 



narrow yellow-green part of the spectrum near 5560 A is used. For low 



o 



intensities the focus shifts to the blue-green near 5040 A. The colors 

 having longer and shorter wavelengths form blurred disks of light of 

 relatively low intensity on top of the sharply focused image. With an 

 increase in the diameter of the pupil, the periphery of the lens is exposed 



