PROPERTIES OF THE RETINA. 353 



After-images may be used in a very instructive way to show that 

 our estimates of the size of a retinal image vary with the distance to 

 which we project it, that is, with the distance at which we suppose 

 we see it. Once the image is, so to speak, branded on the retina, 

 its actual size, of course, does not vary, but our judgment of its size 

 may be made to vary rapidly by projecting the image upon screens at 

 different distances. If, for instance, in obtaining the after-image 

 of the strips shown in Fig. 147 one moves the white paper used 

 to catch the image toward and away from the eye, the apparent size 

 varies proportionally to its distance. 



Color Contrasts. By color contrast is meant the influence 

 that one color field has upon a contiguous one. If, for instance, a 

 piece of blue paper is laid upon a larger yellow square, the color 

 of each of them is heightened by contrast. A piece of blue 

 paper on a blue background does not appear so saturated as when 

 placed against a yellow background. The influences of contrast 

 may be shown in a great variety of ways.* For instance, if a disc 

 like that in the illustration, Fig. 149A, is rotated rapidly, it should 

 give circles of gray, the darkest at the middle; but each circle should 

 be uniform as it is made by the fusion of a definite amount of white 

 and black. On the contrary, the appearance obtained is that repre 



Fig. 1494. Black and white disc for ex- Fig. 149. Showing the result when the 

 periment on contrast. (Rood.) disc A is set into rapid rotation. (flood.) 



sented in Fig. 1495. Each circle appears darker on its outer edge 

 where it borders on a lighter circle, and lighter on its inner edge where 

 it borders on a darker circle. Similar contrasts may be obtained from 

 comparing shadows cast by yellow and white light. If a rod be 

 arranged in a dark room so as to cast a shadow from an opening 

 admitting daylight and one also from a lighted candle, either shadow 

 taken separately appears black, but if the two are cast side by side 

 one will appear blue, the other yellow. The shadow cast by the 

 daylight, being illuminated by the yellow candle-light, will appear 

 yellow, and the other shadow, that from the candle-light, will by 



* Rood, "Modern Chromatics," " International Scientific Series." 

 23 



