22. Sir Isaac Newton observes, that he had never been able to produce a perfect white by the mixture of only two primary colours, and seems to doubt whether such a white can be compounded even of three. He tells us that one part of red lead, and five parts of verdigris, composed a dun colour, like that of a mouse; but there is nothing in all this which militates against the explanation here given of the cause of the coloured shadows of bodies; for even supposing that there did not exist in nature any two bodies of such colours as to form perfect whiteness by their mixture; or, to go still further, supposing that no two prismatic colours of the sun could form a compound perfectly white; still the facts and reasonings here stated respecting the mixtures of such colours as are called contrasts, are so near the truth, that they furnish a satisfactory account of the appearances of the colours of the shadows which we have been considering. The terms by which we are accustomed to denominate colours, have not a very accurate or precise meaning, and particularly those terms which denote colours that are known to be mixtures of others, as green, purple, and orange: neither the prismatic green, nor the colour of any known green body, may, perhaps, combine with red so as to make actually an accurate white, and yet the existence or composition of such a green may not be impossible. The philosophical reader will clearly perceive, that no argument of any weight can be drawn from considerations of this sort against this theory of coloured shadows.