COLOR THEORIES
A full review of the theories held about color is not necessary in a
work of this nature, and those who have more time for and further
interest in the subject will find mentioned in Appendix B to this volume
the titles of a number of admirable works and treatises.
The sensation of color is first and preëminently produced by light. But
an electric discharge, internal causes, or even pressure on the eyeball
may also cause it; just how, we do not know. In fact, the whole subject
of color, its causes, and its mechanism, is still in the region of
speculation, although of speculation that may be useful.
Leaving aside the theory of color production by other causes, we will
give our attention to that color sensation caused by the light of the
sun, and briefly to that produced by artificial light.
The cut on page 14 shows the construction of the eye viewed from the
side. We see that light enters the front of the eye through the cornea
and lens and strikes the interior coating, which is the retina. This is
a wonderful membrane, very thin, but composed, as we see in the next
illustration, magnified many times (page 15), of a marvellous network
made of minute nerves and blood vessels ending on the innermost surface
in tiny rods and cones. These rods and cones in some mysterious way are
acted upon by light, and, like the outposts of an army, send messages of
form and color to the brain. Color is therefore spoken of as “an
internal sensation,” and is fine or poor as are the eyes and brain of
the person who sees it.
[Illustration:
THE CONSTRUCTION OF THE HUMAN EYE AS VIEWED FROM THE SIDE.
(Nearly life size.)
]
What is light, we ask? Scientists answer that it is something which
comes to us from a luminous or light-giving body. Sir Isaac Newton
pronounced it to consist of fine atoms moving toward us rapidly. A later
theory is called the _wave theory_—that there exists throughout space a
fine impalpable medium, “the light-bearing ether,”—that this ether moves
in waves, which, beating upon the retinas of our eyes as ocean waves
beat upon the shore, produce what we call _light_.
[Illustration:
CROSS-SECTION OF THE RETINA, SHOWING THE RODS AND CONES.
(Very much magnified.)
]
Sunlight compared to candle or gas light appears to be white; this white
was proved by Sir Isaac Newton in 1672 to consist of many colors
combined in one ray. He was the first to divide such a ray of sunlight,
which he did by letting it fall through a slit in the window of a
darkened room, then through a prism, or three-sided piece of glass, on
white paper. If this experiment be repeated there will be seen “a long
streak of pure and beautiful colors which blend into each other by
gentle gradations.” Anyone who has seen a rainbow has seen the same
separation of colors, as the raindrops act in the same way as the prism
and divide the rays of sunlight into their component colors.
The “spectrum” is the name given to the streak of colors when produced
by the help of the prism, and it and the rainbow contain the same colors
in the same order. The experiment has also been made of passing this
streak of colors through a second prism, when they again unite and the
ray of simple white light reappears.
An instrument called a “spectroscope” has been invented, and is
constantly used by scientific students of color, which analyzes a ray of
light still better than the simple prism. With its aid, early in this
century, Wollaston and Fraunhofer discovered that the spectrum of
sunlight, in addition to its colors, was crossed by many fine, dark,
fixed lines. These have been named Fraunhofer lines, and are most useful
in dividing and mapping out the limits of the different colors. Still a
later invention called a “diffraction grating,” made either of speculum
metal or of glass silvered on the back and ruled with fine parallel
lines, sometimes as many as eighteen thousand to the English inch, is
used in place of a prism. With the use of improved methods Professor
Rowland of Johns Hopkins University has made one ruled with some fifty
or sixty thousand lines. A ray of sunlight can be divided by this
without the disadvantage of crowding the colors in the middle, as is
unavoidable by the wedge-shaped glass of the prism.
Plate II shows a solar spectrum as produced by a prism and also one as
shown by a diffraction grating. They both give the colors and the main
Fraunhofer lines, the latter being numbered.
Although not essential to the practical use of this manual, we will now
return to the theories of the primary colors, so called, upon which
differing views have been held. Sir David Brewster’s theory of three
primaries—red, yellow, and blue—has been the most popular, because of
the ease with which the three so-called secondary colors may be made by
mixing paint of the three primaries, as follows: red and blue, violet;
blue and yellow, green; yellow and red, orange. Artists have generally
adopted it; Chevreul, the great director of the Gobelin tapestries,
based his whole color system on the theory of three primary colors—red,
yellow, and blue; three secondary colors made by combinations of the
first three—orange, green, and violet; and three tertiary colors made
from combinations of the second three—olive, russet, and citrine. We
must, however, discriminate carefully between pigments, paints, and
light. By experiment we prove that yellow and blue light do not make
green, but white; that red and green light make yellow; and so on, so
that the theory of Thomas Young is now more generally followed by
scientists. As Rood gives it in his _Modern Chromatics_, “there can be
in an objective sense no such thing as three fundamental colors, or
three primary kinds of colored light. In a totally different sense,
however, something of this kind is not only possible, but, as the recent
advances of science show, highly probable. We have already seen in a
previous chapter that in the solar spectrum the eye can distinguish no
less than a thousand different hues. Every small, minute, almost
invisible portion of the retina possesses this power, which leads us to
ask whether each atom of the retina is supplied with an immense number
of nerve fibrils for the reception and conveyance of this vast number of
sensations.
[Illustration:
DIAGRAM ILLUSTRATING THE YOUNG-HELMHOLTZ THEORY OF COLOR
SENSATION.
]
“According to the theory of the celebrated Thomas Young, each minute
elementary portion of the retina is capable of receiving and
transmitting three different sensations; or we may say that each
elementary portion of its surface is supplied with three nerve fibrils,
adapted for the reception of three sensations. One set of these nerves
is strongly acted on by long waves of light and produces the sensation
we call red; another set responds most powerfully to waves of medium
length, producing the sensation we call green; finally, the third set is
strongly stimulated by short waves, and generates the sensation known as
violet.” (This might perhaps rather be called violet blue, as scientists
differ as to the exact shade.) “The red of the spectrum, then, acts
powerfully on the first set of these nerves; but according to Young’s
theory, it also acts on the two other sets, but with less energy. The
same is true of the green and violet rays of the spectrum; they each act
on all three sets of nerves, but most powerfully on those specially
designed for their reception.” All this will be better understood by the
aid of the accompanying diagram, which is taken from Helmholtz’s great
work, _Physiological Optics_. In this figure, along the horizontal lines
1, 2, 3 are placed the colors of the spectrum properly arranged, and the
curves above them indicate the degree to which the three kinds of nerves
are acted on by these colors. Thus we see that nerves of the first kind
are powerfully stimulated by red light, are much less affected by
yellow, still less by green, and very little by violet light. Nerves of
the second kind are much affected by green light, less by yellow and
blue, still less by red and violet. The third kind of nerves answer
readily to violet light, and are successively less affected by other
kinds of light in the following order: blue, green, yellow, orange, red.
The next point in the theory is that if all three sets of nerves are
simultaneously stimulated to about the same degree the sensation which
we call white will be produced. This result would almost lead us into
calling white a color—and the most brilliant one of all. These are the
main points of Young’s theory, which was published as long ago as 1802,
and more fully in 1807. Attention has been called to it within the last
few years by Helmholtz, and it is mainly owing to his labors and those
of Maxwell that it now commands such respectful attention. Thus far the
study of color-blindness has furnished evidence in favor of the theory
of Young, and its phenomena are more easily explained by this than by
any other theory.
A recent invention by Frederick E. Ives of Philadelphia has also been
cited in its support. Through the use of what he calls a
photo-chromoscopic camera he takes through three color screens—a red, a
green, and a blue one—three negatives. These negatives, placed in an
instrument called by him a stereo-photo-chromo-scope (which resembles a
stereoscope, and which also holds three screens of the same colors),
produce to the eyes an image so perfect in color and relief that “people
have been seen to place their hand in front of it before they were
convinced that they did not see a direct reflection.” Various sets of
three hues, or modified hues, might be used to produce the same effect.
In 1878, having re-investigated the subject thoroughly, Hering published
in Vienna a paper advocating another theory. According to this “the
retina is provided with three visual substances, and the fundamental
sensations are not three, but six,—
Black and white,
Red and green,
Blue and yellow.
Each of these three pairs corresponds to an assimilation or
diassimilation process in one of the visual substances; thus red light
acts on the red-green substance in exactly the opposite way from green
light, and when both kinds of light are present in suitable proportions
a balance is effected, and both sensations, red and green, vanish.”[2]
One of the latest accounts of these theories (of Young-Helmholtz and
Hering), written in English, is to be found in Dr. Foster’s _Text-book
of Physiology_. It contains a full and clear discussion of the merits
and demerits of both theories from a scientific standpoint. From it we
give the accompanying diagram illustrating Hering’s theory of color
vision.
[Illustration:
DIAGRAM ILLUSTRATING HERING’S THEORY OF COLOR SENSATION.
]
Edridge Green also discusses both theories fully in connection with
color-blindness.
On one point all these theories agree, which is that perfect or normal
color vision is made up of three factors, or as Foster says, it is
“_tri-chromic_, based on three or the equivalent of three primary
sensations.” The first, the Brewster theory, states that they are red,
yellow, and blue colors; the second, the Young-Helmholtz theory, that
there are three kinds of nerve fibrils in the retina, affected
respectively by red, blue, and green, and their combinations of the
spectrum; while that of Hering is that in the eye there are three
changeable visual substances which are increased or diminished
accordingly as the rays of black and white, yellow and blue, or red and
green, fall upon them.
Le Conte, in his work _Sight_, says of the latter part of this
theory, “according to Hering, complementary colors are the result of
opposite affections of the retina, so that there are only two
essentially distinct color affections of the retina, which, with
their opposites, produce two pairs of complementary colors; the one
with its opposite produces red and green; the other with its
opposite, yellow and blue. This, though more doubtful, seems a
probable cause of complementariness.” Also, “Stanley Hall ...
believes that color is perceived by the cones (in the retina) alone;
further, that different parts of the same cone vibrate with
different degrees of rapidity, and therefore respond to different
colors, and the conical form is adapted for this purpose. In order
to gain a clearer conception we may imagine each cone to be made up
of a number of buttons of graduated sizes joined together. These
buttons, on account of their different sizes, would vibrate with
different degrees of rapidity, and therefore co-vibrate with
different colors. White light, he supposes, vibrates the whole
series; red light the thicker, and violet the thinner portion of the
series; or, taking Hering’s view of the primary colors, we may
imagine that red and green rays affect one portion and yellow and
blue rays another portion of the same cone.”
From the fact that in 1876 F. Boll discovered that the retina contained
a red or purple substance that quickly disappeared on exposure to light,
Kuhne elaborated, after further experiments with light upon that
substance, a still later theory of color vision which supposes that the
light waves produce in the retina different compounds that give rise to
the sensation of the different colors.
Mrs. Franklin of Baltimore has lately given us a theory of “light
sensation,” as she prefers to call it, which has been favorably
received.[3] The question of the specific uses of the rods and cones in
the retina has been a puzzling one, and she suggests that they may be of
the same nature, but in different stages of development,—in other words,
that the rods are undeveloped cones. As there are more cones than rods
in the middle of the retina, and as color is seen more vividly there,
the inference is that the cones are susceptible to both light and color,
while the rods are only sensitive to light. Such a theory seems to
explain the results of many experiments heretofore made by scientists.
Some discussion of the subtile and beautiful colors produced by
interference, refraction, absorption, and polarization, as well as by
opalescence, fluorescence, and phosphorescence, might aptly follow here,
but that such discussion hardly comes within the scope of this mainly
practical book. Readers who wish to understand and experiment with them
are referred to the works of Rood, Church, and Dove.