[From the American Journal of Science and Arts, Yol. XLl.—Mar., 1866.] 
On the Green Tint produced by mixing Blue and Yellow 
Powders. 
0. N. ROOD, 
Professor of Physics in Columbia Cdlfege. 
It is known to every one, that the mechanical mixture of 
yellow and blue powders, (“water or oil colors,”) produces a 
more or less lively green. This fact, with others similar in 
nature, was formerly used by some physicists in support of the 
view that there are only three primary colors—red, yellow and 
blue,—and that the other tints of the spectrum are formed by a 
mixture of these three in certain proportions. Helmholtz has, 
however, shown that the mixture of the blue and yellow of the 
spectrum produces not green, but white light. Similarly it is 
now known that if a circular disc be painted with alternate blue 
and yellow sectors, and then be caused to rotate rapidly, the 
resultant tint will be gray, or reddish gray, but not green. 
Helmholtz has accounted for the production of a green color 
when blue and yellow pigments are mixed, in a manner like the 
following : Light reflected from a painted surface is of a two- 
fold kind: Ist, That which comes directly from the surfaces of 
the little atoms of colored powder; this will, in ordinary day- 
light, be white, and small in amount. 2nd, That light which 
penetrates through two or more atoms of the pigment, and is 
then reflected; this is always colored, and colored merely be- 
cause in its passage through the atoms, certain rays have been 
absorbed. For example, the white light which penetrates 
through an atom of ultramarine, in this operation becomes de- 
prived mainly of the red, orange, and yellow rays; the green, 
blue and violet rays being left comparatively unweakened. 
The particles of chrome-yellow, on the other hand, in the same 
process absorb the blue and violet rays, reflecting back the red, 
orange and green rays. Hence it will be seen that by the joint 
action of the ultramarine and chrome-yellow, all the rays of 
the spectrum are absorbed except the green; therefore green 
light alone is reflected back to the eye. O. N. Rood on the tint from mixing blue and yellow. 
2 
This theory I have lately tested with the aid of the spectro- 
scope. Three strips of white paper, each TVth of an inch in 
breadth, were painted respectively with chrome-yellow, artificial- 
ultramarine, and a mixture of these two pigments in such pro- 
portion as to produce a green. The strips were placed together 
before the slit of a spectroscope, illuminated by light from a 
white cloud, and their spectra compared with a normal spectrum 
from a strip of white paper. It was found that if the intensity 
of the tints of the normal spectrum be represented by the rect- 
angle BX, the spectra from the yellow, green, and blue papers, 
will be approximately represented by the areas contained in the 
curves y, g and B. Mere inspection shows that' the spectrum 
from the green paper is made up of those rays which are com- 
mon to both of the other spectra, its maximum being between 
the fixed lines E and b. Inspection also of the curves given by 
the chrome-yellow and ultramarine, show that both are rather 
deficient in the green element, and therefore comparatively unfit 
to produce a bright green by mixture. 
A disc was now painted with alternate blue and yellow 
sectors, the same materials being used; when it was’ caused to 
revolve rapidly the resultant tint presented to the naked eye 
was slightly reddish grey. Next the same disc was made to 
rotate before the slit of the spectroscope; prismatic analysis 
showed that its spectrum was totally different from the spectrum 
furnished by the green paper: it approximated to the normal 
spectrum, being in fact the sum of the spectra y and B in the 
diagram; there was, however, a rather dark space between the 
fixed lines b and F. This may account for the fact that the re- 
volving disc viewed by the naked eye appeared slightly reddish, 
instead of pure grey. 
It will be seen, then, that the results of spectral analysis ac- 
count for this singular phenomenon in all its details. 
It would be equally easy to account for the fact, that much 
more brilliant greens are produced by mechanically mixing 
those yellow and blue pigments, which are, one or both, origin- 
ally slightly greenish in tint. 
New York, March 10, 1866.