142 Royal Society :— 
results of the present investigation, it appears that the colours of the 
spectrum, as laid down according to Newton’s method from actual 
observation, lie, not in the circumference of a circle, but in the 
periphery of a triangle, showing that all the colours of the spectrum 
may be chromatically represented by three, which form the angles of 
this triangle. 
Wave-length in millionths of Paris inch. 
DeHPlebe Hak Sere 2328, about one-third from line C to D, 
Green........ 1914, about one-quarter from E to F, 
Blue ........ 1717, about half-way from F to G. 
The theory of three primary colours has been often proposed as 
an interpretation of the phenomena of compound colours, but the 
relation of these colours to the colours of the spectrum dees not 
seem to have been distinctly understood till Dr. Young (Lectures on 
Natural Philosophy, Kelland’s edition, p. 345) enunciated his theory 
of three primary sensations of colour which are excited in different 
proportions when different kinds of light enter the organ of vision. 
According to this theory, the threefold character of colour, as perceived 
by us, is due, not to a threefold composition of light, but to the 
constitution of the visual apparatus which renders it capable of being 
affected. in three different ways, the relative amount of each sensation 
being determined by the nature of the incident light. If we could 
exhibit three colours corresponding to the three primary sensations, 
each colour exciting one and one only of these sensations, then since 
all other colours whatever must excite more than one primary sensa- 
tion, they must find their places in Newton’s diagram within the 
triangle of which the three primary colours are the angles. 
Hence if Young’s theory is true, the complete diagram of all colour, 
as perceived by the human eye, will have the form of a triangle. 
The colours corresponding to the pure rays of the spectrum must 
all lie within this triangle, and all colours in nature, being mixtures 
of these, must lie within the line formed by the spectrum. If 
therefore any colours of the spectrum correspond to the three pure 
primary sensations, they will be found at the angles of the triangle, 
and all the other colours will lie within the triangle. 
The other colours of the spectrum, though excited by uncom- 
pounded light, are compound colours; because the light, though 
simple, has the power of exciting two or more colour-sensations in 
different proportions, as, for imstance, a blue-green ray, though not 
compounded of blue rays and green rays, produces a sensation com- 
pounded of those of blue and green. 
The three colours found by experiment to form the three angles 
of the triangle formed by the spectrum on Newton’s diagram, may 
correspond to the three primary sensations. 
A different geometrical representation of the relations of colour 
may be thus described. ‘Take any point not in the plane of Newton’s 
diagram, draw a line from this point as origin through the point 
representing a given colour on the plane, and produce them so that 
the length of the line may be to the part ent off by the plane as the 
intensity of the given colour is to that of the corresponding point on 
