BERING'S THEORY OF COLOR VISION. 527 



still seen as a bright band on a uniform grayish field. 

 Here there is no contrast to produce the error of judgment, 

 and from this and other experiments Hering concludes that 

 light acting on one part of the retina produces inverse 

 changes in all the rest, and that this has an important part 

 in producing the phenomena of contrasts. Similar pheno- 

 mena may be observed with colored objects; in their nega- 

 tive after-images each tint is represented by its complemen- ' 

 tary, as black is by white in colorless vision. 



Endeavoring to exclude such loose general explanations 

 as " errors of judgment/' Hering proposes a theory of vision 

 which can only be briefly sketched here. We may put all 

 our colorless sensations in a continuous series, passing 

 through grays from the deepest black to the brightest 

 white; somewhere half-way between will be a neutral 

 gray which is as black as it is white. We may do some- 

 thipof similar with our color sensations; as in gray we see 

 black and white so in purple we see red and blue, and all 

 cokrs containing red and blue may be put in a series of 

 which one end is pure red, the other pure blue. So with red 

 and yellow, blue and green, yellow and green. If we call to 

 mind the whole solar spectrum from yellow to blue, through 

 the yellow-greens, green, and blue-greens, we get a series in 

 which all but the terminals have this in common that they 

 contain some green. Green itself forms, however, a special 

 point; it differs from all tints on one side of it in contain- 

 ing no yellow, and from all on the other in containing no 

 blue. In ordinary language this is recognized: we give 

 it a definite name of its own and call it green. Its sim- 

 plicity compared with the doubleness of its immediate 

 neighbors entitles it to a distinct place in the color-sensa- 

 tion series. There are three other color sensations which 

 like green are simple and must have specific names of their 

 own; they are red, blue, and yellow. Green may be pure 

 green or yellow green or blue green, but never yellow 

 and bluish at once, or reddish. Eed may be pure or 

 yellowish or bluish, but never greenish. Eed and green 

 are thus mutually exclusive; yellow and blue stand in a 

 similar relationship. All other color sensations, as orango 



