Some years ago Sir Charles Wheatstone drew my attention to a work by Christian Ernst Wuensch, Leipzig 1792, in which the author announces the proposition that there are neither five nor seven, but only three simple colours in white light.  Wuensch produced five spectra, with five prisms and five small apertures, and he mixed the colours first in pairs, and afterwards in other ways and proportions.  His result is that red is a simple colour incapable of being decomposed; that orange is compounded of intense red and weak green; that yellow is a mixture of intense red and intense green; that green is a simple colour; that blue is compounded of saturated green and saturated violet; that indigo is a mixture of saturated violet and weak green; while violet is a pure simple colour.  He also finds that yellow and indigo blue produce white by their mixture.  Yellow mixed with bright blue (Hochblau) also produces white, which seems, however, to have a tinge of green, while the pigments of these two 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