THE HERING THEORY
In the second half of the 
nineteenth century, a second 
school of thought arose parallel 
to, but until recently seemingly 
irreconcilable with, the Young-
Helmholtz color theory. Ewald 
Hering (1834-1918) interpreted 
the results of color mixing by 
proposing the existence, in the 
eye, brain, or both, of three 
opponent processes, one for 
red-green sensation, one for 
yellow-blue, and a third, 
qualitatively different from the 
first two, for black-white. 
Hering was impressed by the 
nonexistence of any colors--
and the impossibility of even 
imagining any colors--that 
could be described as 
yellowish-blue or reddish-
green and by the apparent 
mutual canceling of blue and 
yellow or of red and green when 
they are added together in the 
right proportions, with 
complete elimination of hue--
that is, with the production of 
white. Hering envisioned the 
red-green and yellow-blue 
processes as independent, in 
that blue and red do add to give 
bluish-red, or purple; similarly 
red added to yellow gives 
orange; green added to blue 
gives bluish-green; green and 
yellow gives greenish-yellow. 
In Hering's system, yellow, 
blue, red, and green could be 
thought of as "primary" colors. 
Anyone looking at orange can 
imagine it to be the result of 
mixing red and yellow, but no 
one looking at red or blue would 
see it as the result of mixing 
any other colors. (The feeling 
that some people have that 
green looks like yellow added to 
blue is probably related to their 
childhood experience with 
paint boxes.) Hering's notions 
of red-green and yellow-blue 
processes seemed to many to be 
disconcertingly dependent on 
intuitive impressions about 
color, but it is surprising how 
good the agreement is among 
people asked to select the point 
on the spectrum where blue is 
represented, untainted by any 
subjective trace of green or 
yellow. The same is so for 
yellow and green. With red, 
subjects again agree, this time 
insisting that some violet be 
added to counteract the slight 
yellowish appearance of long-
wavelength light. (It is this 
subjective red that when added 
to green gives white; ordinary 
(spectral) red added to green 
gives yellow.)