584 ANIMAL PHYSIOLOGY. 
mixture of the wave-lengths of red and yellow. Again, certain 
colors known as complementary by psychic fusion gave rise 
to white, though no physical mixture of such colored pig- 
ments will produce white. These are red and blue-green; 
orange and blue; yellow and indigo-blue; green-yellow and 
violet. 
Now, when a child beholds orange, he has not the faintest 
idea that it is related to red, or that white can be in any way 
produced from any combination of colors, any more than, when 
he hears a perfect musical chord, has he any idea of its being 
produced by the simultaneous production of its component 
notes. To him both the colors and the chord are independent, 
facts. But by simple experiments their origin may be illus- 
trated. As regards comple- 
mentary colors, Lambert’s ex- 
periment may easily be per- 
formed: Place a red wafer (or 
a slip of paper) on a sheet of 
white paper, and about three 
inches behind it a blue one, 
Hold a plate of glass be- 
tween the two and vertically, 
Fie. 426.—Lambert’s experiment. The wafers so that while gazing at the 
oT class vinte at a Cites Borsa “T «red wafer through it a re- 
flected image of the blue one 
will be thrown into the eye in the same direction as that of the 
red image, the result being a sensation of purple. 
As before referred to, a rotating disk on which all the colors 
of the spectrum are represented in equal subdivisions, when the 
speed is sufficiently great, appears white from the fusion of the 
sensations. Of course, instead of all the colors, complementary 
ones suffice. As a matter of fact, we may recognize six funda- 
mental colors—white, black, red, yellow, green, and blue—and 
these may be the outcome of the physiological mixture of three 
“standard ” sensations. 
We now proceed to matters of speculation. At the present 
day two theories to account for color-vision monopolize atten- 
tion: 1. The Young-Helmholtz theory assumes that there are 
only three primary sensations, or, in other words, that the reti- 
na is affected only by rays of light corresponding to red, green, 
and violet (or blue); and the manner in which any color is pro- 
duced (in the mind) will appear from an examination of Fig. 
427, Thus, when red is the color seen, though the retinal 
