



V: 



OPTICAL DEFECTS OF THE EYE. 503 



merits 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 cau.se colored borders to ap- 

 pear around the edges of the images of objects. In the eye 

 TVG 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 is inexact they appear 

 .also when the boundary between a white and a black sur- 

 face 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 con- 

 taining all the rays of intermediate refrangibility. Ordi- 

 dinary 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 the sunlight be suffered to enter through the 

 hole, it will be found that with one accommodation (that 

 lor 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 centre 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. 

 122, taken a little in front of r r. This defect is found in all 



