VISION AND THE TECHNIQUE OF ART. 9 



of a white wedge on a black background and a black wedge on a white 

 background. The base of the wedges was one-fourth inch and their 

 distance six feet from the camera. 



The top picture was taken through a red, the middle through a 

 yellow, and the bottom one through a blue filter. The camera was 

 focused to give a sharp image with the yellow filter. The top picture 

 shows the kind of image that is formed on the retina by the red light, 

 the middle one the kind of image that is formed by the yellow light and 

 the bottom one the kind of image that is formed by the blue light. If 

 you imagine these superimposed, which is what takes place on the 

 retina, the combined picture will have a blue diffused edge extending 

 over the black and a less wide red edge. The color of the white near 

 the edges will be slightly yellowish due to the subtraction of the blue 

 and red. 



With a lens free from chromatic aberration no such effect is pro- 

 duced. This is shown by Figure 8, which is a photograph of the same 

 objects taken at the same distance and in the same way, with a cor- 

 rected lens. The images of all the wedges in this case are sharp and 

 clear and of the same size. They will all exactly superimpose and no 

 chromatic edges will be formed. 



Under ordinary circumstances unless the attention is especially 

 called to them these chromatic rings and edges formed in the eye are 

 not seen. This is due, it is believed, partly to the fact that the red 

 rings overlie the green, which being complementary colors form white 

 light, and to the fact that the blue is so spread out that it is relatively 

 weak. However, if one looks carefully for these rings or edges they 

 can be seen around an arc light at night, in the blue haze or halo on the 

 dark background. 



The red and blue chromatic circles or edges can be seen separately 

 by looking at a dark object, such as a window sash against a bright sky 

 at a distance of three to six feet and shutting off the light from half the 

 pupil by passing a card or piece of paper close to the eye, the edge of 

 the paper being kept parallel to the window sash. One side of the 

 frame will have a red orange edge, the other a bluish edge. If the 

 card is brought in from the other direction the color of the edges will 

 reverse. Without the card the colors overlie each other and become 

 much less visible for the reason given above. However, once the 

 phenomenon has been noticed a soft floating purplish edge becomes 

 apparent even without the card. 



A very striking example of chromatic aberration, and one which 

 gives a very good idea of its magnitude, is apparent when one looks 



