THE EYE AS AN OPTICAL INSTRUMENT. 527 



admirably as a physiological instrument, is by no means per- 

 fect 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 refran- 

 gible than those at the red end. Hence they are brought to a 

 focus sooner. The light emanating from a point on a white 

 object 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 accommo- 

 dated so as to bring to a focus on the retina parallel red rays, 

 then violet rays from the same source will meet half a milli- 

 meter 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 

 combining together lenses made of different kinds of glass; 

 such compound 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 observed. 

 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 proceeding from a point in front, even when these are 

 all of the same refrangibility. Convex lenses whose surfaces 

 are segments of spheres, as are those of the eye, bring to a 

 focus sooner the rays which pass through their marginal than 



