o colours when mixed
always give a more or less beautiful green, Wuensch very emphatically
distinguishes the mixture of pigments from that of lights. Speaking of
the generation of yellow, he says, 'I say expressly _red and green
light_, because I am speaking about light-colours (Lichtfarben), and
not about pigments.' However faulty his theories may be, Wuensch's
experiments appear in the main to be precise and conclusive. Nearly
ten years subsequently, Young adopted red, green, and violet as the
three primary colours, each of them capable of producing three
sensations, one of which, however, predominates over the two others.
Helmholtz adopts, elucidates, and enriches this notion. (_Popular
Lectures_, p. 249. The paper of Helmholtz on the mixture of colours,
translated by myself, is published in the _Philosophical Magazine_ for
1852. Maxwell's memoir on the Theory of Compound Colours is published
in the _Philosophical Transactions_, vol. 150, p. 67.)]

[Footnote 9: The following charming extract, bearing upon this point,
was discovered and written out for me by my deeply lamented friend Dr.
Bence Jones, when Hon. Secretary to the Royal Institution:--

'In every kind of magnitude there is a degree or sort to which our
sense is proportioned, the perception and knowledge of which is of
the greatest use to mankind. The same is the groundwork of
philosophy; for, though all sorts and degrees are equally the object
of philosophical speculation, yet it is from those which are
proportioned to sense that a philosopher must set out in his
inquiries, ascending or descending afterwards as his pursuits may
require. He does well indeed to take his views from many points of
sight, and supply the defects of sense by a well-regulated
imagination; nor is he to be confined by any limit in space or time;
but, as his knowledge of Nature is founded on the observation of
sensible things, he must begin with these, and must often return to
them to examine his progress by them. Here is his secure hold: and
as he sets out from thence, so if he likewise trace not often his
steps backwards with caution, he will be in hazard of losing his way
in the labyrinths of Nature.'--(_Maclaurin: An Account of Sir I.
Newton's Philosophical Discoveries. Written 1728; second edition_,
1750; pp. 18, 19.)
]

[Footnote 10: I do not wish to encumber the conception here with the
details of the