60 
PEOFESSOE J. CLEEE MAXWELL OX 
Sir David Beewstee has given a diagram of three curv'es, in which the base-line repre- 
sents the length of the spectrum, and the ordinates of the curves represent, by estima- 
tion, the intensities of the three kinds of light at each point of the spectrum. I have 
employed a diagram of the same kind to express the results arrived at in this paper, the 
ordinates being made to represent the intensities of each of the three elements of colour, 
as calculated from the experiments. 
The most complete series of experiments on the mixture of the colours of the spec- 
trum, is that of Professor Helmholtz* of Konigsberg. By using two slits at right 
angles to one another, he formed two pure spectra, the fixed lines of which were seen 
crossing one another when viewed in the ordinary way by means of a telescope. The 
colom’S of these spectra were thus combined in every possible way, and the effect of the 
combination of any two could be seen separately by drawing the eye back from the 
eyepiece of the telescope, when the compound colour was seen by itself at the eye-hole. 
The proportion of the components was altered by turning the combined slits round in 
their own plane. 
One result of these experiments was, that a colour, chromatically identical with 
white, could be formed by combining yellow with indigo. M. Helmholtz was not then 
able to produce white with any other pair of simple colours, and considered that three 
simple colours were required in general to produce white, one from each of the three 
portions into which the spectrum is divided by the yellow and indigo. 
Professor GEASSMANNf showed that Newton’s theory of compound colours implies 
that there are an infinite number of pairs of complementary colours in the spectrum, 
and pointed out the means of finding them. He also showed how colours may be 
represented by lines, and combined by the method of the parallelogram. 
In a second memoir J, M. Helmholtz describes his method of ascertaining these pairs 
of complementary colours. He formed a pure spectrum by means of a slit, a prism, and 
a lens ; and in this spectrum he placed an apparatus having two parallel slits which 
were capable of adjustment both in position and breadth, so as to let through any two 
portions of the spectrum, in any proportions. Behind this slit, these rays were united 
in an image of the prism, which was received on paper. By arranging the slits, the 
colour of this image may be reduced to white, and made identical with that of paper 
illuminated with white light. The wave-lengths of the component colours were then 
measured by observing the angle of diffraction through a grating. It was found that 
the colours from red to green-yellow (A= 2082 ) were complementary to colours ranging 
from green-blue (A= 1818 ) to violet, and that the colours between green-yellow and green- 
blue have no homogeneous complementaries, but must be neutralized by mixtures of red 
and violet. 
M. Helmholtz also gives a provisional diagram of the curve formed by the spectrum on 
Newton’s diagram, for which his experiments did not furnish him with the complete data. 
* PoGGENDOEri'’s Annalen, Band Ixxxvii. (Philosophical Magazine, 1852, December.) 
t PoGGEXDOsri'’s Annalen, Band Ixxxix. (Philosophical Magazine, 1854, April.) J Ibid. Band xciv. 
