14 
NATURE 
[May 4, 1871 

green light, as you see. In the same way most blue and yellow 
paints, when mixed, appear green. ‘The light which falls on the 
mixture is so beaten about between the yellow particles and the 
blue, that the only light which survives is the green. But 
yellow and blue light when mixed do not make green, as you 
will see if we allow them to fall on the same part of the screen 
together. 
It is a striking illustration of our mental processes that many 
persons have not only gone on believing, on the evidence of the 
mixture of pigments, that blue and yellow make green, but that 
they have even persuaded themselves that they could detect the 
separate sensations of blueness and of yellowness in the sensation 
of green. 
We have availed ourselves hitherto of the analysis of light by 
coloured substances. We must now return, still under the 
guidance of Newton, to the prismatic spectrum. Newton not 
only 
Untwisted all the shining robe of day, 
but showed how to put it together again. We have here a pure 
spectrum, but instead of catching it on a screen, we allow it to pass 
through a lens large enough to receive all the coloured rays. 
These rays proceed, according to well-known principles in optics, 
to form an image of the prism on a screen placed at the proper 
distance. This image is formed by rays of all colours, and you 
see the result is white. But if I stop any of the coloured rays, 
the image is no longer white, but coloured ; and if I only let 
through rays of one colour, the image of the prism appears of 
that colour. 
I have here an arrangement of slits by which I can select one, 
two, or three portions of the light of the spectrum, and allow 
them to form an image of the prism while all the rest are stopped. 
This gives me a perfect command of the colours of the spectrum, 
and I can produce on the screen every possible shade of colour 
by adjusting the breadth and the position of the slits through 
which the light passes. I can also, by interposing a lens in the 
passage of the light, show you a magnified image of the slits, by 
which you will see the different kinds of light which compose the 
mixture. 
The colours are at present red, green, and blue, and the mix- 
ture of the three colours is, as you see, nearly white. Let us try 
the effect of mixing two of these colours. Ked and blue form a 
fine purple or crimson, green and blue form a sea-green or sky- 
blue, red and green form a yellow. 
Here again we have a fact not universally known. No painter, 
wishing to produce a fine yellow, mixes his red with his green. 
The result would bea very dirty drab colour. He is furnished by 
nature with brilliant yellow pigments, and he takes advantage of 
these. When he mixes red and green paint, the red light scattered 
by the red paint is robbed of nearly all its brightness by getting 
among particles of green, and the green light fares no better, 
for it is sure to fall in with particles of red paint. But when the 
pencil with which we paint is composed of the rays of light, the 
effect of two coats of colour is very different. The red and the 
green form a yellow of great splendour, which may be shown to 
be as intense as the purest yellow of the spectrum. 
I have now arranged the slits to transmit the yellow of the 
spectrum. You see it is similar in colour to the yellow formed 
by mixing redand green. It differs from the mixture, however, 
in being strictly homogeneous in a physical point of view. The 
prism, as you see, does not divide it into two portions as it did 
the mixture. Let us now combine this yellow with the blue of 
the spectrum. The result is certainly not green; we may make 
it pink if our yellow is of a warm hue, but if we choose a 
greenish yellow we can produce a good white. 
You have now seen the most remarkable of the combinations 
of colours—the others differ from them in degree, not in kind. I 
must now ask you to think no more of the physical arrangements 
by which you were enabled to see these colours, and to concen- 
trate your attention upon the colours you saw, that is to say, on 
certain sensations of which you were conscious. We are here 
surrounded by difficulties of a kind which we do not meet with 
in purely physical inquiries. We can all feel these sensations, 
but none of us can describe them. They are not only private 
property, but they are incommunicable. We have names for the 
external objects which excite our sensations, but not for the 
sensations themselves. 
When we look at a broad field of uniform colour, whether it 
is really simple or compound, we find that the sensation of 
colour appears to our consciousness as one and indivisible. We 

cannot directly recognise the elementary sensations of which it 
is composed, as we can distinguish the component notes of a 
musical chord. <A colour, therefore, must be regarded as a single 
thing, the quality of which is capable of variation. 
To bring a quality within the grasp of exact science, we must 
conceive it as depending on the values of one or more variable 
quantities, and the first step in our scientific progress is to deter- 
mine the number of these variables which are necessary and suffi- 
cient to determine the quality of a colour. We do not require 
any elaborate experiments to prove that the quality of colour can 
vary in three and only in three independent ways. 
One way of expressing this is by saying, with the painters, that 
colour may vary in hue, tint, and shade. : 
The finest example of a series of colours varying in hue, is the 
spectrum itself. A difference in hue may be illustrated by the 
difference between adjoining colours in the spectrum. The series 
of hues in the spectrum is not complete; for, in order to get 
purple hues, we must blend the red and the blue. 
Tint may be defined as the degree of purity of a colour. 
Thus, bright yellow, buff, and cream-colour, form a series of 
colours of nearly the same hue, but varying in tint. The tints 
corresponding to any given hue form a series, beginning with the 
most pronounced colour, and ending with a perfectly neutral tint. 
Shade may be defined as the greater or less defect of illumina- 
tion. If we begin with any tint of any hue, we can form a 
gradation from that colour to black, and this gradation is a series 
of shades of that colour, Thus we may say that brown is a dark 
shade of orange. 
The quality of a colour may vary in three different and inde- 
pendent ways. We cannot conceive of any others. In fact, if 
we adjust one colour to another, so as to agree in hue, in tint, 
and in shade, the two colours are absolutely indistinguishable. 
There are therefore three, and only three, ways in which a colour 
can vary. 
I have purposely avoided introducing at this stage$ of our in- 
quiry anything which may be called a scientific experiment, in 
order to show that we may determine the number of quantities 
upon which the variation of colour depends by means of our 
ordinary, experience alone. 
Here is a point in this room : if I wish to specify its position, 
I may do so by giving the measurements of three distances— 
namely, the height above the floor, the distance from the wall 
behind me, and the distance from the wall at my left hand. 
This is only one of many ways of stating the position of a 
point, but it is one of the most convenient. Now, colour also 
depends on three things. If we call these the intensities of the 
three primary colour sensations, and if we are able in any way 
to measure these three intensities, we may consider the colour as: 
specified by these three measurements. Hence the specification 
of a colour agrees wiih the specification of a point in the room 
in depending on three measurements. 
Let us go a step farther, and suppose the colour sensations: 
measured on some scale of intensity, and a point found for which 
the three distances, or co-ordinates, contain the same number of 
feet as the sensations contain degrees of intensity. .Then we 
may say, by a useful geometrical convention, that the colour is 
represented to our mathematical imagination by the point so 
found in the room; and if there are several colours, represented. 
by several points, the chromatic relations of the colours will be 
represented by the geometrical relations of the points. This 
method of expressing the relations of colours is a great help to 
the imagination. You will find these relations of-colours stated 
in an exceedingly clear manner in Mr, Benson’s ‘‘ Manual of 
Colour,” one of the very few books on colour in which the state- 
ments are founded on legitimate experiments. 
There is a still more convenient method of representing the 
relations of colours, by means of Young’s triangle of colours. 
It is impossible to represent on a plane piece of paper every 
conceivable colour, to do this requires space of three dimen- 
sions. If, however, we consider only colours of the same shade, 
that is, colours in which the sum of the intensities of the three 
sensations is the same, then the variations in tint and in hue of 
all such colours may be represented by points ona plane. For 
this purpose we must draw a plane cutting off equal lengths 
from the three lines representing the primary sensations. ‘The 
part of this plane within the space in which we have been distri- 
buting our colours will be an equilateral triangle. The three 
primary colours will be at the three angles, white or gray will 
be in the middle, the tint or degree of purity of any colour will 
be expressed by its distance from the middle point, and its hue 
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