PEOFESSOE J. CLEEK MAXWELL OX 
in the spectrum cannot be so resolved. This result seems to put an end to the preten- 
sion of yellow to be considered a primary element of colour. 
In the same way the colours from the primary gi’een to blue are chromatically iden- 
tical with mixtures of these ; and the extreme ends of the spectrum are probably equi- 
valent to mixtures of red and blue, but they are so feeble in illumination that experi- 
ments on the same plan with the rest can give no result, but they must be examined by 
some special method. When observations have been obtained from a greater number 
of individuals, including those whose vision is dichromatic, the chart of the specti’um 
may be laid down independently of accidental differences, and a more complete discus- 
sion of the laws of the sensation of colour attempted. 
POSTSCRIPT. 
Eeceived May 8, — Eead May 24, 1860. 
Since sending the above paper to the Royal Society, I have .obtained some observa- 
tions of the colour of the spectrum by persons whose vision is “ dichromic,” and who 
are therefore said to be “ colour-blind.” 
The instrument used in making these observations was similar in principle to that 
formerly described, except that, in order to render it portable, the rays are reflected 
back through the prisms, nearly in their original du-ection ; thus rendering one of the 
limbs of the instrument unnecessary, and allowing the other to be shortened consider- 
ably on account of the greater angular dispersion. The principle of reflecting light, so as 
to pass twice through the same prism, was employed by me in an instrument for com- 
bining colours made in 1856, and a reflecting instrument for obserring the spectrum 
has been constructed independently by M. PoREO. 
Light from a sheet of paper illuminated by sunlight is admitted at the slits X, Y. Z 
(hg. 8, Plate II.), falls on the prisms P and P' (angles=45°), then on a concave silvered 
glass, S, radius 34 inches. The light, after reflexion, passes again through the prisms 
P' and P, and is reflected by a small mirror, e, to the slit E, where the eye is placed to 
receive the light compounded of the colours corresponding to the positions and breadths 
of the slits X, Y, and Z. 
At the same time, another portion of the light from the illuminated paper enters the 
instrument at BC, is reflected at the mirror M, passes through the lens L, is reflected at 
the mirror M', passes close to the edge of the prism P, and is reflected along uTth the 
coloured light at e, to the eye-slit at E. 
In this way the compound colour is compared mth a constant white light in optical 
juxtaposition with it. The mirror M is made of silvered glass, that at M' is made of 
glass roughened and blackened at the back, to reduce the intensity of the constant light 
to a convenient value for the experiments. 
