THE EYE AS A SENSORY APPARATUS. 549 



be noticed not only that the light band between the squares 

 is intensely white, much more so than the normal idio-retinal 

 light, but, as the image fades away, often the two dark after- 

 images of the squares disappear entirely with all of the 

 corona, except that part between them which is 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 phe- 

 nomena of contrasts. Similar phenomena may be observed 

 with colored objects; in their negative after-images each tint 

 is represented by its complementary, 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 stated here. We may put all 

 our colorless sensations in a continuous series, passing through 

 grays from the deepest black to the brightest white; some- 

 where half-way between will be a neutral gray which is as 

 black as it is white. We may do something similar with our 

 color sensations; as in gray we see black and white so in 

 purple we see red and blue, and all colors 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 spec- 

 trum from yellow to blue, through the yellow-greens, green, 

 and blue -greens, we get a series in which all but the ter- 

 minals 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 containing 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 simplicity compared with the doubleness 

 of its immediate neighbors entitles it to a distinct place in 

 the color-sensation series. There are three other color sensa- 

 tions 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. Red may be pure or 

 yellowish or bluish, but never greenish. Red and green are 

 thus mutually exclusive; yellow and blue stand in a similar 



