VISION AND THE TECHNIQUE OF ART. 33 



for blue was then made sufficiently longer so that the color rendering 

 would be normal in the center of the picture while on the outer parts 

 the blue would be stronger in approximate accordance with the results 

 found by Abney. A lens having the approximate aberrations and 

 distortion of the eye was used. 



The results were very interesting. The effect was two-fold; first: 

 to render all the colors outside of the "yellow spot" bluer; second: 

 to make more apparent the aberrations in the images formed by oblique 

 rays and thus cause a greater softening of the outer parts of the picture. 

 In the photographs in which the color rendering all over the picture 

 was the same as we get in our yellow spot the aberrations of light 

 from the blue end of the spectrum, although they existed, were so low 

 in intensity as to produce no effect. When, however, the intensity of 

 the blue light was made greater these aberrations became apparent. 



These effects are primarily chromatic and show very poorly in black 

 and white reproductions. The difference in the apparent aberration 

 however is evident in the photographs shown in Figures 39a, b, and c. 

 These were taken with a lens having the approximate aberrations of 

 the eye. In Figure 39 (a) no filter used. In Figure 39 (b) a filter 

 having the approximate absorption of our yellow spot covered the 

 entire picture. In Figure 39 (c), which represents the conditions we 

 get in our eye, a similar filter covered an angular area corresponding to 

 that of the yellow spot. 



It will be seen that the whole of the picture in Figure 39 (a) is much 

 softer than that in 39 (b). This is due to the greater amount of the 

 aberrated blue light which struck the plate in Figure 39 (a) and which 

 is absorbed by the filter used in taking 39 (b). In Figure 39 (c) the 

 center portions are sharp due to the local action of the yellow filter 

 while the outer part is soft due to its absence. It is very evident that 

 Figure 39 (c) which approximates the conditions we get on our retina 

 is much more pleasing 14 than Figure 39 (a) or 39 (b). Figure 39 (d) 

 is a photograph of the same scene taken with a corrected lens. This 

 seems to be pretty conclusive evidence that the purpose of the yellow 

 spot is to counteract the strong chromatic aberration in the eye by 

 reducing the brightness of the light from the blue end of the spectrum 

 so that it ceases to be apparent. 



14 It is regretted that a very marked effect that is apparent in the original 

 photograph is lost in the reproductions. 



