VISION AND THE TECHNIQUE OF ART. 7 



than the yellow bundle and that the yellow bundle lies to the left of 

 the red. 



The eye normally focuses for yellow light, i.e., so that the small 

 cross section of the yellow bundle falls on the retina. If the eye looks 

 at red, yellow, and blue point sources at the same time the retina 

 would be in the position relative to the three bundles as shown in 

 Figure 3. It will be noted that the smallest cross section of the blue 

 bundle lies in front of the retina, that of the red behind. 



St.l. 



Axi. Red 



A«» Ye 



Figure 3. Image bundles formed by light from red, yellow, and blue point 

 sources. Their displacement right and left shows the Chromatic Aberration 

 of the eye. The different distances from the retina at which the individual 

 rays, in each bundle, cut the axis shows the Spherical Aberration. 



It will also be noted that where the red and blue bundles cut the 

 retina they are much larger in diameter than the yellow bundle. This 

 is also shown in Figure 4 which is a photograph of the magnified image 

 of a point source of white light taken with a lens which has approxi- 

 mately the same chromatic aberration as the eye. A white light source 

 is composed of light of all wave lengths. The camera was focused to 



