VISION AND THE TECHNIQUE OF ART. 13 



sharply at the same time a blue light about three feet away and a red 

 light at about twenty feet away. 



We described in Chapter I the characteristic chromatic diffusion 

 circles and edges of an object in focus. We saw that a white light 

 point source as at A, Figure 10, would form an image with a yellowish 

 center surrounded by red and blue diffusion circles. A white light 

 point source at B, Figure 10, however, will form quite a different image. 

 The blue in the white light being in focus will, as we have just shown, 

 form a sharp image. The yellow will be out of focus and form a yellow 

 diffusion circle around the blue and the red will be still more out of 

 focus and form a larger red diffusion circle which will extend outside 

 that of the yellow. 



Figure 11, is a photograph of a point source of white light taken with 

 a lens which has approximately the same chromatic aberration as the 

 eye. The lens wasfocused to give a sharp image of the yellow rays in a 

 white light source about six feet away. The pictures are of a white 

 light point source about three feet away. As in Figure 4 the top 

 image was taken through a red, the middle one through a yellow, and 

 the bottom one through a blue filter. The combined image which is 

 the appearance the white light point source would have to the eye is 

 markedly different from that shown in Figure 4. 



A white light point source at R, Figure 10, will form a still different 

 kind of image. The red in the white light being in focus will form a 

 sharp image. The yellow will be out of focus and form a yellow 

 diffusion circle around the red and the blue being still more out of 

 focus will form a larger blue diffusion circle. 



Figure 12 is a photograph similar to those described in Figure 4 and 

 Figure 11, but with the objects twenty feet away. The images 

 formed are very different from those shown in Figure 4 and 1 1 . 



Images of white light point sources situated at other distances along 

 the axis will vary from those shown above, their characteristics de- 

 pending on the position of the fixation point and their distance from it. 



A lens corrected for chromatic aberration forms very different 

 images of similar white light point sources. Figures 13 and 14 are 

 photographs taken with a corrected lens. The photographs were 

 made in the same way as those shown in Figures 11 and 12 of white 

 light point sources in similar positions. As the point in the focus of a 

 corrected lens is in focus for all colors so a point out of focus is out of 

 focus for all colors and to the same extent. The diffusion circles for 

 red, yellow and blue light in Figures 13 and 14 are therefore all about 

 the same size. The combined image although enlarged and fuzzy will 



