316 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Chromatic Aberration. — In the above account of the dioptric apparatus 

 of the eye the phenomena have been described as they would occur with mono- 

 chromatic light — i. c with light having but one degree of refrangibility. But 

 the light of the sun is composed of an infinite number of rays of different 

 degrees of refrangibility. Hence when an image is formed by a simple lens 

 the more refrangible rays — i. c. the violet rays of the spectrum — are brought 

 to a focus sooner than the less refrangible red rays. The image therefore 

 appears bordered by fringes of colored light. This phenomenon of chromatic 

 aberration can be well observed by looking at objects through the lateral por- 

 tion of a simple lens, or, still better, by observing them through two simple 

 lenses held at a distance apart equal to the sum of their focal distances. The 

 objects will appear inverted (as through an astronomical telescope) and sur- 

 rounded with borders of colored light. Now, the chromatic aberration of the 

 eve is so slight that it is not easily detected, and the physicists of the eighteenth 

 century, in their efforts to produce an achromatic lens, seem to have been 

 impressed by the fact that in the eye a combination of media of different 

 refractive powers is employed, and to have sought in this circumstance an 

 explanation of the supposed achromatism of the eye. Work directed on this 

 line was crowned with brilliant success, for by combining two sorts of glass of 

 different refractive and dispersive powers it was found possible to refract a ray 

 of light without dispersing it into its different colored rays, and the achromatic 

 lens, thus constructed, became at once an essential part of every first-class opti- 

 cal instrument. Now, as there is not only no evidence that the principle of 

 the achromatic lens is employed in the eye, but distinct evidence that the eye 

 is uncorrected for chromatic aberration, we have here a remarkable instance of 

 a misconception of a physical fact leading to an important discovery in physics. 

 The chromatic aberration of the eye, though so slight as not to interfere at all 

 with ordinary vision, can be readily shown to exist by the simple experiment 

 of covering up one half of the pupil and, looking at a bright source of light 

 e. g. a window. If the lower half of the pupil be covered, the cross-bars of 



Fig. 137.— Diagram to illustrate chromatic aberration. 



the window will appear bordered with a fringe of blue light on the lower and 

 reddish light on the upper side. The explanation usually given of the way in 

 which tin- result is produced is illustrated in Fig. 137. Owing to the chromatic 

 aberration of the eye all the rays emanating from an object at A are not 

 focussed accurately on the retina, but if the eye is accommodated for a ray of 

 medium refrangibility, the violet rays will be brought to a focus in front of 

 the retina at V, while the red rays will be focussed behind the retina at R. 



