as seen in fig. 18, in which bfea is the spectrum of the slit ab,
and cefd that of the slit cd; the coloured spectra are contained
Fig. 18.—Diagram of Double Spectrum
partially superposed.
in the triangle gef, and
by arrangement, the
effects of mixture of any
two simple colours may
be observed.
(b) By Method of
Reflection.—Place a red
wafer on b in fig. 19 and
a blue wafer on d, and
so angle a small glass plate a as to transmit to the eye a
reflection of the blue wafer on d in the same line as the rays
Fig. 19.—Diagram showing
Lambert's Method of mixing Sensations
of Colour.
transmitted from the red
wafer on b. The sensation
will be that of purple; and
by using wafers of different
colours, many experiments
may thus be performed.
(c) By Rotating Disks which quickly superpose on the same Area of Retina the Impressions of Different Wave-lengths.—Such disks may be constructed of cardboard, on which coloured sectors are painted, as shown in fig. 20, representing diagrammatically the arrangement of Sir Isaac Newton. The angles of the sectors were thus given by him:—
| Red | 60° 45.5′ |
| Orange | 34° 10.5′ |
| Yellow | 54° 41′ |
| Green | 60° 45.5′ |
| Blue | 54° 41′ |
| Indigo | 34° 10.5′ |
| Violet | 60° 45.5′ |
With sectors of such a size, white will be produced on rotating
the disk rapidly. This method has been carried out with great
efficiency by the colour-top of J. Clerk-Maxwell. It is a flat top,
Fig. 20.—Diagram of the Colour
Disk of Sir Isaac Newton.
on the surface of which disks
of various colours may be
placed. Dancer has added
to it a method by which, even
while the top is rotating
rapidly and the sensation of
a mixed colour is strongly
perceived, the eye may be
able to see the simple colours
of which it is composed.
This is done by placing on
the handle of the top, a
short distance above the
weighted a little on one side.
coloured surface, a thin black
disk, perforated by holes of
various size and pattern, and
weighteed a little on one side. This disk vibrates to and fro
rapidly, and breaks the continuity of the colour impression;
and thus the constituent colours are readily seen.
3. The Geometric Representation of Colours.—Colours may be arranged in a linear series, as in the solar spectrum. Each point of the line corresponds to a determinate impression of colour; the line is not a straight line, as regards luminous effect, but is better represented by a curve, passing from the red to the violet. This curve might be represented as a circle in the circumference of which the various colours might be placed, in which case the complementary colours would be at the extremities of the same diameter. Sir Isaac Newton arranged the colours in the form of a triangle, as shown in fig. 21. If we place three of the spectral colours at three angles, thus—green, violet and red—the sides of the triangle include the intermediate colours of the spectrum, except purple.
The point S corresponds to white, consequently, from the
intersection of the lines which join the complementary colours, the
straight lines from green to S, RS and VS represent the amount of
green, red and violet necessary to form white; the same holds good
for the complementary colours; for example, for blue and red, the
line SB = the amount of blue, and the line SR = the amount of red
Fig. 21.—Geometrical Representation
of the Relations of Colours as shown
by Newton.
required to form white.
Again, any point, say M,
on the surface of the
triangle, will represent a
mixed colour, the composition
of which may be
obtained by mixing the three
fundamental colours in the
proportions represented by
the length of the lines M to
green, MV and MR. But
the line VM passes on to
the yellow Y; we may then
replace the red and green
by the yellow, in the
proportion of the length of the
line MY, and mix it with violet in the proportion of SV. The
same colour would also be formed by mixing the amount MY of
yellow with MS of white, or by the amount RM of red with the
amount MD of greenish blue.
The following list shows characteristic complementary colours, with their wave-lengths (λ) in millionths of a millimetre:—
Red, λ 656.
Orange, λ 608.
Gold-yellow, λ 574.
Yellow, λ 567.
Greenish yellow, λ 564.
Blue-green, λ 492.
Blue, λ 490.
Blue, λ 482.
Indigo-blue, λ 464.
Violet, λ 433.
By combining colours at opposite ends of the spectrum, the effect of the intermediate colours may be produced; but the lowest and the highest, red and violet, cannot thus be formed. These are therefore fundamental or primary colours, colours that cannot be produced by the fusion of other colours. If now to red and violet we add green, which has a rate of vibration about midway between red and violet, we obtain a sensation of white. Red, green and violet are therefore the three fundamental colours.
4. Physiological Characters of Colours.—Colour physiologically is a sensation, and it therefore does not depend only on the physical stimulus of light, but also on the part of the retina affected. The power of distinguishing colours is greatest when they fall on, or immediately around, the yellow spot, where the number of cones is greatest. In these regions more than two hundred different tints of colour may be distinguished. Outside of this area lies a middle zone, where fewer tints are perceived, mostly confined to shades of yellow and blue. If intense coloured stimuli are employed, colours may be perceived even to the margin of the periphery of the retina, but with weak stimuli coloured objects may seem to be black, or dark like shadows. In passing a colour from the periphery to the centre of the yellow spot, remarkable changes in hue may be observed. Orange is first grey, then yellow, and it only appears as orange when it enters the zone sensitive to red. Purple and bluish green are blue at the periphery, and only show the true tint in the central region. Four tints have been found which do not thus change: a red obtained by adding to the red of the spectrum a little blue (a purple), a yellow of 574.5 λ, a green of 495 λ and a blue of 471 λ.
The question now arises, How can we perceive differences in colour? We might suppose a molecular vibration to be set up in the nerve-endings synchronous with the undulations of the luminiferous aether, without any change in the chemical constitution of the sensory surface, and we might suppose that where various series of waves in the aether corresponding to different colours act together, these may be fused together, or to interfere so as to give rise to a vibration of modified form or rate that corresponded in some way to the sensation. Or, to adopt another line of thought, we might suppose that the effect of different rays (rays differing in frequency of vibration and in physiological effect) is to promote or retard chemical changes in the sensory surface, “which again so affect the sensory nerves as to give rise to differing states in the nerves and the nerve centres, with differing concomitant sensations.” The former of these thoughts is the foundation of the Young-Helmholtz theory, while the latter is applicable to the theory of E. Hering.
