THE EYE AS AN OPTICAL INSTRUMENT 263 



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 together 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 

 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 those passing through their 

 central parts. If rays proceeding from a point and traversing the 

 lateral part of a lens be brought to a focus at any point, then those 

 passing through the center of the lens will not meet until a little 

 beyond that point. If the retina receive the image formed by the 

 peripheral rays the others will form around this a small luminous 

 circle of light such as would be formed by sections of the cones 

 of converging rays in Fig. 78, taken a little in front of r r. This 

 defect exists in all glass lenses, as it is found impossible in practice 



