8 AMES, PROCTOR AND AMES. 



get as sharp an image as possible of the yellow light and then the three 

 pictures were taken, the top one through a red screen, the middle one 

 through a yellow, 4 the lower one through a blue. The top photo- 

 graph therefore shows the image formed by the red rays in the white 

 light source, the middle one that formed by the yellow rays, and the 

 bottom one that formed by the blue rays. 



Colored images of the same relative difference in size are apparent 

 to the eye looking at a point source through monochromatic screens or 

 filters if the eye is kept focused for yellow. Without any screens or 

 niters the eye receives an image of the nature obtained by combining 

 the three images in Figure 4. It would have a bright center tending 

 towards yellow in color, surrounded by larger and larger rings of 

 shorter and longer wave lengths, the blue rings extending out farther 

 than the red. 



A comparison of the image formed by the eye and that formed by a 

 lens corrected for chromatic aberration is of interest. Such a lens is 

 designed so that light of all wave lengths focuses at the same distance 

 from the lens. This is shown in Figure 5 where it will be seen that the 

 narrowest part of the bundles for light of different wave length, in- 

 stead of being focused at different distances from the lens, all focus at 

 the same distance. Figure 6 is a photograph of a point source of white 

 light taken with a corrected lens. The photographs were made in the 

 same way as those in Figure 4. It will be noticed that where the 

 images taken through the red, yellow and blue screens with the un- 

 corrected lens are all of different size similar images formed by a cor- 

 rected lens are substantially all the same size. That is, this correction 

 causes the images of an object in focus to be much sharper or clearer 

 than those formed in the eye. 



We have been speaking so far only of point source objects. If the 

 object is either a line or an edge, for instance a dark edge of a window 

 against a light sky, the diffusion circles we have been describing take 

 the form of diffusion edges. The distribution of color in these diffu- 

 sion edges follows the same laws as govern that in the diffusion circles. 

 This is shown clearly in Figure 7, which is a photograph taken through 

 a lens having approximately the same chromatic aberration as the eye, 



4 As the filters actually used in taking this and following pictures were East- 

 man Kodak Co. films Red No. 25, Green No. 58 and Blue No. 48, it would be 

 more exact to use the word "green" instead of "yellow." The difference in 

 wave length between the green and yellow however is such that there would be 

 no appreciable differences in the appearance of the photographs whether a 

 green or yellow filter was used. The term yellow will therefore be used for 

 sake of simplicity and clearness. 



