20 AMES, PROCTOR AND AMES. 



This means that with any given fixation, at a given angle of obliquity, 

 there are certain positions in space where tangential black and white 

 lines appear most sharp and other positions where radial black and 

 white lines appear most sharp. It also means that with the same 

 fixation there are still other positions where tangential lines of a 

 particular color will appear most sharp and still other positions where 

 radial lines of the same color appear most sharp. The positions in 

 space therefore where tangential and radial colored lines appear most 

 sharp depend upon their color. 



The different position of the primary and secondary astigmatic 

 fields for different colors has a further effect of causing the images of 

 black and white objects to have characteristic chromatic edges due to 

 their position in space relative to the fixation point. This can be best 

 shown by the following photographs taken with a lens which has 

 approximately the same oblique astigmatism and chromatic aberration 

 as the eye. 



Figure 24 (b) is of a white light point source situated at C, Figure 23, 

 i.e., in the secondary field for yellow at an angular obliquity of about 

 eighteen or twenty degrees. As the lens is focused at the distance 

 marked "fixation point" this is in the plane of the focus. The top 

 picture shows the image formed by the red rays in the white light 

 source; the middle that formed by the yellow; the bottom one that 

 formed by the blue. As the point source is in the secondary field for 

 yellow the yellow light is stretched in a radial direction. Being near 

 the primary field for red the red is beginning to be stretched in a 

 tangential direction. And being beyond the secondary field for blue 

 the blue is stretched in a radial direction forming a diffused radial oval. 



Figure 24 (a) is a photograph of a white light point source situated 

 at B; Figure 23, i.e., in the secondary astigmatic field of red light. 

 The red light is therefore stretched in a radial direction. The point 

 source being beyond the secondary fields for both yellow and blue 

 light they are imaged as diffused radial ovals, the blue more diffused 

 than the yellow. 



Figure 24 (c) is of a white light point source situated at D, Figure 23, 

 i.e., near the primary field of red and yellow and the secondary of blue, 

 consequently we see the red and yellow stretched in a tangential 

 direction, the blue in a radial one. 



The images of white light point sources in similar positions formed 

 by a lens corrected for oblique astigmatism and chromatic aberration 

 are quite different. Figure 24 (e) is such a photograph of a white light 

 point source at C, Figure 23, i.e., in the plane of the focus. The lens 



