TIME-RELATIONS OF RETINAL STIMULATION. 865 



appears in an intense contrast-color. (3) On a rotating white disc are pasted 

 four green sectors, each of which is interrupted in its center by a narrow band of 

 black, concentric with the disc. On rotation of the disc, this ring appears red, and 

 not gray. (4) If a grayish-white surface be looked at with both eyes and a tube about 

 the length and diameter of a finger, made of transparent, colored oiled paper, through 

 the w r alls of which light can pass, be placed in front of one eye, the part of the 

 white surface seen through the tube appears in the contrast-color. The experiment 

 also shows beautifully the contrast in the intensity of illumination. (5) A piece 

 of white paper, with a round, black spot in the middle, when seen through a blue 

 glass, appears blue with a black spot. If a white spot of the same size on a black 

 background be placed in front of the glass, so that its reflected image covers 

 exactly the black spot, it appears in the contrast-color, yellow. (6) The colored 

 shadows also belong to the simultaneous contrasts. "Two conditions are neces- 

 sary for the production of colored shadows first, that the light casting the 

 shadow shall in some manner color the white surface; and second that a second 

 light shall illuminate the shado\v to a certain degree. A short burning candle 

 is placed on a white paper, in the twilight; between them and the diminishing 

 daylight a lead-pencil is placed vertically so that the shadow thrown by the 

 candle is illuminated but not extinguished by the feeble daylight; the shadow 

 will appear of a beautiful blue. That this shadow- is blue will be observed at 

 once : but it is only by close observation that one can convince himself that the w r hite 

 paper acts as a reddish-yellow surface, through the luster of which the blue color 

 is conveyed to the eye. One of the prettiest instances of colored shadows may be 

 seen during the full moon. The light of a candle and that of the moon can be exactly 

 equalized. Both shadows can be made of equal strength and distinctness, so 

 that the two colors completely balance. A board is exposed to the light of the 

 full moon, with the candle a little to one side, and an opaque body is held at a 

 suitable distance in front of the board. A double shadow results, that thrown 

 by the moon and illuminated by the candle-light appearing of an intense reddish- 

 yellow color, while that thrown by the candle and illuminated by the moon appear- 

 ing of a beautiful blue. Where the two shadows coincide and unite to form one 

 a black shadow results (Goethe). (7) The colored reflections are the reverse of 

 the colored shadows. If a piece of silverware be placed near a window, in the 

 twilight, and the light from a candle be allowed to fall on it at the same time, the 

 reflected image of the flame appears yellowish, that of the lessening daylight 

 decidedly blue. (8) A piece of white paper is placed on the table and above it, 

 separated by a horizontal line, a piece of black paper. Now a vertical, black 

 strip is pasted on the white paper and on the black paper a white strip. If these 

 strips are seen through a birefringent spar-prism, each will be doubled, and possess 

 a gray color, because the strip is composed of white and black mixed. The strips 

 on the dark background, however, appear brighter, and those on the white ground 

 darker. Likewise, in an analogous way, with colored strips on a differently col- 

 ored background the experiment shows the contrast-colors beautifully. Landois 

 has found this excellent experiment especially convincing if the objects are cov- 

 ered with translucent tracing paper. 



Some have tried to explain these phenomena as errors of judgment ; in other 

 words, when different impressions act simultaneously, the judgment is so de- 

 ceived that if the action takes place in one position, it will have an extremely 

 slight effect in the neighborhood. Thus, if light affects one portion of the retina, 

 judgment wrongly assumes a slight illumination of the neighboring parts of the 

 retina. The same would be true of colors. The phenomena are, however, 

 much more correctly explained by Hering as true, physiological processes. 

 Partial stimulation by light affects not only the parts so acted upon but also the 

 surrounding retina; the directly stimulated portion by increased dissimilation, 

 the (indirectly stimulated) vicinity by increased assimilation, in such a way, 

 that the latter increase is most pronounced in the immediate vicinity of the 

 illuminated spot, and decreases rapidly with the distance from it. As a result 

 of the increase in assimilation at the part not occupied by the image of the object, 

 the diffused light is ordinarily not perceived. As the increase in assimilation is 

 greatest in the immediate neighborhood of the illuminated spot, the perception 

 of this relatively strong diffused light is largely made impossible. 



On looking for a long time at a dark or a bright object, or at a colored one 

 (for example red) , and then allowing the contrast-effects to appear on the retina, 

 respectively bright or dark, or the contrast-color (green), these appear especially 

 intense. This phenomenon has been designated successive contrast. In this 

 connection the negative after-images obviously take part at the same time. 



55 



