ring
one semicircle of the aperture with a red and the other with a blue or
green glass, the difference between the apparent sizes of the two
semicircles is in my case, and in numerous other cases, extraordinary.
Many persons, however, see the apparent sizes of the two semicircles
reversed. If with a spectacle glass I correct the dispersion of the
red light over the retina, then the blue ceases to give a sharply
defined image. Thus examined, the departure of the eye from
achromatism appears very gross indeed.]

[Footnote 7: Both in foliage and in flowers there are striking
differences of absorption. The copper beech and the green beech, for
example, take in different rays. But the very growth of the tree is
due to some of the rays thus taken in. Are the chemical rays, then,
the same in the copper and the green beech? In two such flowers as the
primrose and the violet, where the absorptions, to judge by the
colours, are almost complementary, are the chemically active rays the
same? The general relation of colour to chemical action is worthy of
the application of the method by which Dr. Draper proved so
conclusively the chemical potency of the yellow rays of the sun.]

[Footnote 8: Young, Helmholtz, and Maxwell reduce all differences of
hue to combinations in different proportions of three primary colours.
It is demonstrable by experiment that from the red, green, and violet
_all_ the other colours of the spectrum may be obtained.

Some years ago Sir Charles Wheatstone drew my attention to a work by
Christian Ernst Wuensch, Leipzig 1792, in which the author announces
the proposition that there are neither five nor seven, but only three
simple colours in white light. Wuensch produced five spectra, with five
prisms and five small apertures, and he mixed the colours first in
pairs, and afterwards in other ways and proportions. His result is
that red is a _simple_ colour incapable of being decomposed; that
orange is compounded of intense red and weak green; that yellow is a
mixture of intense red and intense green; that green is a _simple_
colour; that blue is compounded of saturated green and saturated
violet; that indigo is a mixture of saturated violet and weak green;
while violet is a pure _simple_ colour. He also finds that yellow and
indigo blue produce _white_ by their mixture. Yellow mixed with bright
blue (Hochblau) also produces white, which seems, however, to have a
tinge of green, while the pigments of these tw