Sir J. F. W. Herschel on Colowr-Blindness. 151 
us in pigments, coloured tissues, &e., who have for the most part 
never seen a prismatic spectrum, or at least attended to its pheeno- 
mena. The perceptions of colour afforded by such objects are those 
of white light from which certain rays have been abstracted by ab- 
sorption, that is to say, they are negative hues, or hues of darkness 
rather than of light, inasmuch as all the colouring of the artist is 
based, not on the generation, but on the destruction of light. This 
circumstance, which is not generally recognized, even among edu- 
cated artists, has vitiated all the language of chromatics as applied to 
art, and so placed a barrier between the painter and the photologist, 
which has to be surmounted before they can come to a right under- 
standing of each other’s meaning. It is evident, that, to make 
5 aes on the subject free from this objection, absorptive 
colours must be discarded, at least in bodily mixture with each other. 
Thus it is true that a dingy green may be produced by rubbing 
together in powder prussian blue and the yellow chromate of mer- 
cury above mentioned; but both these agree in reflecting a con- 
siderable, and the latter a very large proportion of green light, to the 
predominance of which in the joint reflected beam its tint is owing. 
So also, when blue and yellow liquids (not acting chemically on each 
other) are mixed, as in water-colour drawings, greens, sometimes 
yery lively ones, are produced. In these cases the yellow absorbs 
almost all the whole of the incident blue, indigo, and violet light, and 
the blue a very large proportion of the red, orange, and yellow, both 
allowing much green to pass ; {and to ¢his, rather than to a mixture of 
the other rays, the resulting tint is due. 
In the light transmitted by cuprate of ammonia of a certain thick- 
ness, the red, orange, yellow, and green are wholly extinguished, 
while the blue, indigo, and violet are allowed to pass. The result is 
the fullest and dluest blue it is possible to obtain. From this result, 
compared with that derived from the analysis of natural yellows, it 
follows that the union on the retina of the yellowest yellow, and the 
bluest blue, in such proportions that neither shall be in excess,jso as 
to tinge the resulting light either yellow or blue, is not green, but 
white. The same conclusion follows from dividing the spectrum 
into two, the one portion containing all the less refrangible rays up 
to the limit of the green and blue, the other all the remaining rays. 
If the blue portion be suppressed, and the remainder reunited by a 
refraction in the opposite direction, the resulting beam is yellow, it 
the other, d/ue, both vivid colours—but if neither, white of course, 
and not green, results from the exact recomposition of the original 
white beam. 
It may be objected to this, that in the complementary colours 
exhibited by doubly-refracted pencils in polarized light, yellow is 
often found to be complementary to purple, and blue to orange. 
But in neither of these pairs of colours is the spectrum divided in 
the manner above indicated ; and, moreover, in many instances yellow 
and blue are found as complementary colours in the oppositely 
polarized peyaias of which examples will be found in the scale of 
tints produced by sulphate of barytes in my paper “ On the Action 
of Crystallized Bodies in Homogencous Light” (Phil. Trans, 1520, 
