FROM C. J. MONRO, Esq.

                                                                Hadley, Barnet, 3d March 1871.

     The Hon. J. W. Strutt, son of Lord Rayleigh, and senior wrangler in 1865, has been meddling with your colours,   and has given occasion also to me to do so again. I send a selection of Natures containing him and me, and my   old contributions of last year, which, or one of which, you say met Mr. Benson's approval. Strutt's last letter ends  with a sentence which obliged me to write to him personally; and I could not help saying, with regard to the   sentence which begins it, p.264, that I thought you would object to inferences founded on comparison by   contrast, and that the proper way was to compare by matching recognized browns with a compound.

     Listing's paper, mentioned in p. 102, was to me rather a paradox,—I had got to regard the subdivisions of the   colour-scale which are assumed in language, as something so arbitrary. If you cared to see it, I have that number   of Poggendorff; I think he would hardly agree with your J. J. Müller. I wish you or Benson could eradicate the   insane trick of reasoning about colours as identified by their names. People seem to think that blue is blue, and  one blue as good as another. Benson's book I have seen (since I heard of it from you), but not read. His way of
mixing by means of a prism is very happy. . . . I wish, with your new set up box, you would just put the prism   observations into relation with the disk ones. It would be very easy. White we have got; and it would only be  strictly necessary to determine two other standard colours, such as vermilion or emerald, by reference to the   spectrum primaries. You don't say whether your dwellers in Mesopotamia and elsewhere agree on the whole   better or worse than "J" and "K," who, I suppose, agree for better and for worse. To judge by their case, the   discrepancy would be a little diminished by taking as units of colour co-ordinates for any given pair of eyes, not
the intensities of the primaries as they appear in the spectrum but their intensities as they appear in the combination  which to that pair of eyes makes (say) white. This amounts to transforming from trilinear to Hamilton's anharmonic  co-ordinates with white for the fourth point,—in the language of "scientific metaphor." On the other hand, ought   not all your co-ordinates to be cooked by multiplying by (d . scale, page 68 ) / ( d . wave-length ) ?

     You know where I learnt scientific metaphor. I have read the address in Section A more than once with much    pleasure, and, I hope, profit in proportion. The pleasure, I [378] confess, was with me, I found it was with   Litchfield, partly that of recognising an old well-remembered style, and reflecting that here at least was something   which might be "thought to be beyond the reach of change." . . . By the way, Boole is "one of the profoundest  mathematicians of our time;" but how about "thinkers"? Certainly his expositions of the principle of a piece of   mathematics are beautiful up to and, I don't doubt, beyond my appreciating. But that last chapter of the Laws,
     etc., from which you quote, with Empedocles and Pseudo-Origen and the rest of them, always seems to me to    render a sound as of a largish internal cavity; and the whole book, taken together with his R.S.E. paper on   testimony and least squares, presents, I think, too many instances of a particular class of fallacy—I know I am  speaking blasphemies, but there would be a strike among the postmen if I put in all the necessary    qualifications—too many instances to be got over, not in absolute number if they were of different kinds, for   anybody may make mistakes, but too many of one kind. The kind is insufficient interpretation, i.e. letting your     equations lead you by the nose. The most serious example,—I maintain it is an example,—is his insisting that his     theory of logic is not founded on quantity, so that it furnishes (he holds) an independent foundation for    probabilities, independent of the usual quantitative foundation. That this is a fallacy, and that in particular it is an    example of the fallacy of insufficient interpretation, is evident surely when you find that, even in the higher case of    "secondary" propositions, the elective symbols represent in his own opinion quantities of "time" after all. With   regard to the sentence you quote, I am always suspicious of any inclination I may feel to find a question too easy;   and, independently of that, your quoting it is itself a staggerer. But the difficulty I confess does strike me as a rather    artificial one. There is nothing, scarcely, in which I think Mill is so right and the Hamiltonians so wrong as that  question about logic being the laws of thought. Hamilton says as thought, Mill says as valid, and so does Boole    and so do you; but if Mill is right, where is the difficulty? Why should the conditions of thinking correctly be   inviolable in the sense of not preventing you from thinking incorrectly, provided they are inviolable in the sense of
ensuring that you take the consequences if you do? The laws of projection in geometry are inviolable, but nobody    ever thought it a paradox that it is possible for a picture to be out of drawing in spite of them, nor is it a paradox   that in unfamiliar classes of cases a rigorously accurate piece of perspective looks out of drawing. Perhaps you  meant, for I suppose the report in Nature is incomplete, that it was a difficulty to say in what sense mathematical   propositions could be said to be certain, considering that one may make mistakes about them. Perhaps something   else, which for the above reason or others, is hidden from me.

     . . . By the way, I hope it is true that you are to profess experimental physics at Cambridge, or what I hope comes   to the same thing, that you are a candidate.