THE EYE AS AN OPTICAL INSTRUMENT 227 



Such eyes are called hypermetropic, or in common language long- 

 sighted. 



Optical Defects of the Eye. The eye, though it answers ad- 

 mirably as a physiological instrument, is by no means perfect 

 optically; not nearly so good, for example, as a good microscope 

 objective. The main defects in it are due to: 



1. Chromatic Aberration. As already pointed out, the rays at 

 the violet end of the solar spectrum are more refrangible than those 

 at the rear end. Hence they are brought to a focus sooner. The 

 light emanating from a point on a white abject does not, therefore, 

 all meet in one point on the retina; but the violet rays come to a 

 focus first, then the indigo, and so on to the red, farthest back of 

 all. If the eye is accommodated so as to bring to a focus on the 

 retina parallel red rays, then violet rays from the same source will 

 meet half a millimeter in front of it, and crossing and diverging 

 there make a little violet circle of diffusion around the red point on 

 the retina. In optical instruments this defect is remedied by com- 

 bining tbgether lenses made of different kinds of glass ; such com- 

 pound lenses are called achromatic. 



The general result of chromatic aberration, as may be seen in a 

 bad opera-glass, is to cause colored borders to appear around the 

 edges of the images of objects. In the eye we usually do not notice 

 such borders unless we especially look for them; but if, while a 

 white surface is looked at, the edge of an opaque body be brought 

 in front of the eye so as to cover half the pupil, colorations will be 

 seen at its margin. If accommodation be inexact they appear also 

 when the boundary between a white and a black surface is ob- 

 served. The phenomena due to chromatic aberration are much 

 more easily seen if light containing only red and violet rays be 

 used instead of white light containing all the rays of intermediate 

 refrangibility. Ordinary blue glass only lets through these two 

 kinds of rays. If a bit of it be placed over a very small hole in an 

 opaque shutter and sunlight be admitted through the hole, it will 

 be found that with one accommodation (that for the red rays) a 

 red point is seen with a violet border, and with another (that at 

 which violet rays are brought to a focus on the retina) a violet 

 point is seen with a red aureole. 



2. Spherical Aberration. It is not quite correct to state that 

 ordinary lenses bring to a focus in one point behind them rays 



