64 
PEOFESSOE J. CLEEK 3IAXWELL OX 
We may conceive an arrangement of actual colours in space founded upon this con- 
struction. Suppose each of these radiating lines representing a given colour to be itself 
illuminated with that colour, the brightness increasing from zero at the origin to unity, 
where it cuts the plane of the diagram, and becoming continually more intense in pro- 
portion to the distance from the origin. In this way every colour in nature may be 
matched, both in quality and quantity, by some point in this coloured space. 
If we take any three lines through the origin as axes, we may, by coordinates 
parallel to these lines, express the position of any point in space. That point will cor- 
respond to a colour which is the resultant of the three colours represented by the three 
coordinates. 
This system of coordinates is an illustration of the resolution of a colour into three 
components. According to the theory of Young, the human eye is capable of three 
distinct primitive sensations of colour, which, by their composition in various propor- 
tions, produce the sensations of actual colour in all their varieties. YTiether any kinds 
of light have the power of exciting these primitive sensations separately, has not yet 
been determined. 
If colours corresponding to the three primitive sensations can be exhibited, then all 
colours, whether produced by light, disease, or imagination, are compounded of these, 
and have their places within the triangle formed by joining the three primaries. If the 
colours of the pure spectrum, as laid down on the diagram, form a triangle, the colours 
at the angles may correspond to the primitive sensations. If the curve of the spectrum 
does not reach the angles of the circumscribing triangle, then no colour in the spec- 
trum, and therefore no colour in nature, corresponds to any of the three primary sensa- 
tions. 
The only data at present existing for determining the primary colom’s, are derived 
from the comparison of observations of colour-equations by colour-bhnd, and by normal 
eyes. The colour-blind equations differ from the others by the non-existence of one of 
the elements of colour, the relation of which to known colours can be ascertained. It 
appears, from observations made for me by two colour-blind persons *, that the element- 
ary sensation which they do not possess is a red approaching to crimson, hung beyond 
both vermilion and carmine. These observations are confirmed by those of Mr. Pole, 
and by others which I have obtained since. I have hopes of being able to procure a set 
of colour-blind equations between the colours of the spectrum, which will indicate the 
missing primary in a more exact manner. 
The experiments which I am going to describe have for their object the determina- 
tion of the position of the colours of the spectrum upon Newton’s diagram, from actual 
observations of the mixtures of those colours. They were conducted in such a way, 
that in every observation the judgment of the observer was exercised upon two parts of 
an illuminated field, one of which was so adjusted as to be chromatically identical udth 
the other, which, during the whole series of observations, remained of one constant 
* Transactions of the Eoyal Society of Edinburgh, vol. xxi. pt. 2. p. 286. 
