card, the rectangle is seen red: cutting off the less refrangible
fringe, the rectangle is seen blue. By means of a thin glass prism
(W), I deflect one portion of the colours, and leave the residual
portion. On the screen are now two coloured rectangles produced in
this way. These are _complementary_ colours--colours which, by their
union, produce white. Note, that by judicious management, one of these
colours is rendered _yellow_, and the other _blue_. I withdraw the
thin prism; yellow and blue immediately commingle, and we have _white_
as the result of their union. On our way, then, we remove the fallacy,
first exposed by Wünsch, and afterwards independently by Helmholtz,
that the mixture of blue and yellow lights produces green.

Restoring the circular aperture, we obtain once more a spectrum like
that of Newton. By means of a lens, we can gather up these colours,
and build them together, not to an image of the aperture, but to an
image of the carbon-points themselves.

Finally, by means of a rotating disk, on which are spread in sectors
the colours of the spectrum, we blend together the prismatic colours
in the eye itself, and thus produce the impression of whiteness.

Having unravelled the interwoven constituents of white light, we have
next to inquire, What part the constitution so revealed enables this
agent to play in Nature? To it we owe all the phenomena of colour, and
yet not to it alone; for there must be a certain relationship between
the ultimate particles of natural bodies and white light, to enable
them to extract from it the luxury of colour. But the function of
natural bodies is here _selective_, not _creative_. There is no colour
_generated_ by any natural body whatever. Natural bodies have showered
upon them, in the white light of the sun, the sum total of all