May 4, 1871] 
NATURE 
15 

will depend on the angular position of the line which joins it 
with the middle point. 
Thus the ideas of tint and hue can be expressed geometrically 
on Youny’s triangle. To understand what is meant by shade, 
_ we have only to suppose the illumination of the whole triangle 
increased or diminished, so that by means of this adjustment of 
illumination Young’s triangle may be made to exhibit every 
variety of colour. If we now take any two colours in the 
triangle and mix them in any proportions, we shall find the re- 
sultant colour in the line joining the component colours at the 
point corresponding to their centre of gravity. 
I have said nothing about the nature of the three primary sen- 
sations, or what particular colours they most resemble. In order 
to lay down on paper the relations between actual colours, it is 
not necessary to know what the primary colours are. We may 
take any three colours, provisionally, as the angles of a triangle, 
and determine the position of any other observed colour with 
respect to these, so as to form a kind of chart of colours. 
Of all colours which we see, those excited by the different 
rays of the prismatic spectrum have the greatest scientific impor- 
tance. All light consists either of some one kind of these rays, 
or of some combination of them. The colours of all natural 
bodies are compounded of the colours of the spectrum. If, 
therefore, we can form a chromatic chart of the spectrum, ex- 
pressing the relations between the colours of its different por- 
tions, then the colours of all natural bodies will be found within 
a certain boundary on the chart defined by the positions of the 
colours of the spectrum. 
But the chart of the spectrum will also help us to the knowledge 
of the nature of the three primary sensations. Since every sensa- 
tion is essentially a positive thing, every compound colour-sensation 
must be within the triangle of which the primary colours are the 
angles. In particular, the chart of the spectrum must be entirely 
within Young’s triangle of colours, so that if any colour in the 
spectrum is identical with one of the colour-sensations, the chart 
of the spectrum must be in the form of a line having a sharp 
angle at the point corresponding to this colour. 
J have already shown you how we can make a mixture of any 
three of the colours of the spectrum, and vary the colour of the 
mixture by altering the intensity of any of the three components. 
If we place a compound colour side by side with any other 
colour, we can alter the compound colour till it appears exactly 
similar to the other. This can be done with the greatest exact- 
ness when the resultant colour is nearly white. I have therefore 
constructed an instrument which I may call a colour-box, for the 
purpose of making matches between two colours. It can only 
be used by one observer at a time, and it requires daylight, so I 
have not brought it with me to-night. It is nothing but the 
realisation of the construction of one of Newton’s propositions 
in his “ Lectiones Opticze,” where he shows how to take a beam 
of light, to separate it into its components, to deal with these 
components as we please by means of slits, and afterwards to 
unite them into a beam again. The observer looks into the bex 
through a small slit. He sees a round field of light, consisting 
of two semicircles divided by a vertical diameter. The semi- 
circle on the left consists of light which has been enfeebled by 
two reflexions at the surface of glass. That on the right is a 
mixture of colours of the spectrum, the positions and intensities 
of which are regulated by a system of slits. 
The observer forms a judgment respecting the colours of the 
two semicircles. Suppose he finds the one on the right hand 
redder than the other, he says so, and the operator, by means of 
screws outside the box, alters the breadth of one of the slits, so 
as to make the mixture less red ; and so on, till the right semi- 
circle is made exactly of the same appearance as the left, and the 
line of separation becomes almost invisible. 
When the operator and the observer have worked together for 
some time they get to understand each other, and the colours are 
adjusted much more rapidly than at first. 
When the match is pronounced perfect, the positions of the 
slits, as indicated by a scale, are registered, and the breadth of 
each slit is carefully measured by means of a gauge. The 
registered result of an observation is called a ‘‘ colour equation.” 
It asserts that a mixture of three colours is, in the opinion of the 
observer (whose name is given), identical with a neutral tint, 
which we shall call Standard White. Each colour is specified 
by the position of the slit on the scale, which indicates its position 
in the spectrum, and by the breadth of the slit, which is a measure 
of its intensity. 
In order to make a survey of the spectrum we select three 

points for purposes of comparison, and we call these the three 
Standard Colours. The standard colours are selected on the 
same principles as those which guide the engineer in selecting 
stations for a survey. They must be conspicuous and invariable, 
and not in the same straight line. 
In the chart of the spectrum you may see the relations of the 
various colours of the spectrum to the three standard colours, and 
to each other. It is manifest that the standard green which I 
have chosen cannot be one of the true primary colours, for the 
other colours do not all lie within the triangle formed by joining 
them. But the chart of the spectrum may be described as con- 
sisting of two straight lines meeting in a point. This point 
corresponds to a green about a fifth of the distance from 4 towards 
F. This green has a wave-length of about 510 millionths of a 
millimetre by Ditscheiner’s measure. This green is either the 
true primary green, or at least it is the nearest approach to it 
which we can ever see. Proceeding from this green towards the 
red end of the spectrum, we find the different colours lying al- 
most exactly in a straight line. This indicates that any colour 
is chromatically equivalent to a mixture of any two coiours on 
opposite sides of it and in the same straight line. The extreme 
red is considerably beyond the standard red, but it is in the same 
straight line, and therefore we might, if we had no other evidence, 
assume the extreme red as the true primary red. We shall see, 
however, that the true primary red is not exactly represented in 
colour by any part of the spectrum. It lies somewhat beyond 
the extreme red but in the same straight line. 
On the blue side of primary green the colour equations are 
seldom so accurate. The colours, however, lie in a line which 
is nearly straight. I have not been able to detect any measurable 
chromatic difference between the extreme indigo and the violet. 
The colours of this end of the spectrum are represented by a 
number of points very close to each other. We may suppose 
that the primary blue is a sensation differing little from that ex- 
cited by the parts of the spectrum near G, 
Now, the first thing which occurs to most people about this 
result is that the division of the spectrum is by no means a fair 
one. Between the red and the green we have a series of 
colours apparently very different from either, and having such 
marked characteristics that two of them, orange and yellow, 
have received separate names. The colours between the green 
and the blue, on the other hand, have an obvious resemblance to 
one or both of the extreme colours, and no distinct names for 
these colours have ever become popularly recognised. 
I do not profess to reconcile this discrepancy between ordinary 
and scientific experience. It only shows that it is impossible, by 
a mere act of introspection, to make a true analysis of our sen- 
sations. Consciousness is our only authority ; but consciousness 
must be methodically examined in order to obtain any trustworthy 
results. 
I have here, through the kindness of Professor Huxley, a 
picture of the structure upon which the light falls at the back of 
the eye. There is a minute structure of bodies like rods and 
cones or pegs, and it is conceivable that the mode in which we 
become aware of the shapes of things is by a consciousness which 
differs according to the particular rods on the ends of which the 
light falls, just as the pattern on the web formed by a Jacquard 
loom depends on the mode in which the perforated cards act on 
the system of movable rods in that machine. In the eye we 
have on the one hand light falling on this wonderful structure, 
and on the other hand we have the sensation of sight. We can- 
not compare these two things ; they belong to opposite categories. 
The whole of Metaphysics lies like a great gulf between them. 
It is possible that discoveries in physiology may be made by 
tracing the course of the nervous disturbance 
Up the fine fibres to the sentient brain; 
but this would make us no wiser than we are about those colours 
sensations which we can only know by feeling them ourselves. 
Still, though it is impossiBle to become acquainted with a sensa- 
tion by the anatomical study of the organ with which it is con- 
nected, we may make use of the sensation as a means of investi- 
gating the anatomical structure. 
A remarkable instance of this is the deduction of Helmholtz’s 
theory of the structure of the retina from that of Young with 
respect to the sensation of colour. Young asserts that there are 
three elementary sensations of colour; Helmholtz asserts that 
there are three systems of nerves in the retina, each of which has 
for its function, when acted on by light or any other disturbing 
agent, to excite in us one of these three sensations, 
