THE THEORY OF COMPOUND COLOURS. 
81 
right side of the above Table, and are represented by the curves of fig. 9, Plate II., where 
the left-hand curve represents the intensity of the “ yellow ” element, and the right-hand 
curve that of the “ blue ” element of colour as it appears to the colour-blind. 
The appearance of the spectrum to the colour-blind is as follows : — 
From A to E the colour is pure “ yellow ” very faint up to D, and reaching a maximum 
between D and E. From E to one-third beyond F towards G the colour is mixed, 
varying from “ yellow ” to “ blue,” and becoming neutral or “ white ” at a point near F. 
In this part of the spectrum, the total intensity, as given by the dotted line, is decidedly 
less than on either side of it, and near the line F, the retina close to the “ yellow spot ” 
is less sensible to light than the parts fuidher from the axis of the eye. This peculiarity 
of the light near F is even more marked in the colour-blind than in the ordinary eye. 
Beyond F fhe “ blue ” element comes to a maximum between F and G, and then dimi- 
nishes towards H ; the spectrum from this maximum to the end being pure “ blue.” 
In fig. 10, Plate II. these results are represented in a different manner. The point D, 
corresponding to the sensation wanting in the colour-blind, is taken as the origin of coordi- 
nates, the “ yellow ” element of colour is represented by distances measured horizontally 
to the right from D, and the “ blue ” element by distances measured vertically from 
the horizontal line through D. The numerals indicate the different colours of the 
spectrum according to the scale shown in fig. 9, and the coordinates of each point indi- 
cate the composition of the corresponding colour. The triangle of colours is reduced, 
in the case of dichromic vision, to a straight line “ B ” “ Y,” and the proportions of 
“ blue ” and “ yellow ” in each colour are indicated by the ratios in which this line is 
cut by the line from D passing through the position of that colour. 
The results given above were all obtained with the light of white paper, placed in 
clear sunshine. I have obtained similar results, when the sun was hidden, by using the 
light of uniformly illuminated clouds, but I do not consider these observations sufficiently 
free from disturbing circumstances to be employed in calculation. It is easy, however, 
by means of such observations, to verify the most remarkable phenomena of colour- 
blindness, as for instance, that the colours from red to green appear to differ only in 
brightness, and that the brightness may be made identical by changing the width of the 
slit ; that the colour near F is a neutral tint, and that the eye in viewing it sees a dark 
spot in the direction of the axis of vision ; that the colours beyond are all blue of 
different intensities, and that any “ blue ” may be combined with any “ yellow ” in such 
proportions as to form "‘white.” These results I have verified by the observations of 
another colour-blind gentleman, who did not obtain sunlight for his observations ; and 
as I have now* the means of carrying the requisite apparatus easily, I hope to meet with 
other colour-blind observers, and to obtain their observations under more favourable 
circumstances. 
On the Comparison of Colour -hlhid with ordinary Vision hy means of Observations 
with Coloured Papers. 
In March 1859 I obtained a set of observations by Mr. Simpson, of the relations 
