TO C. J. MONRO, Esq.

                                         Glenlair, Dalbeattie, 15th March 1871.

     I have been so busy writing a sermon on Colour, and Tyndalising my imagination up to the lecture point, that along    with other business I have had no leisure to write to any one.

     I think a good deal may be learned from the names of colours, not about colours, of course, but about names;   and I think it is remarkable that the rhematic instinct has been so much more active, at least in modern times, on  the less refrangible side of primary green (lambda = 510 x 10e-9 inches).

     I am not up in ancient colours, but my recollection of the interpretations of the lexicographers is of considerable   confusion of hues between red and yellow, and rather more discrimination on the blue side. Qu. If this is true, has    the red sensation become better developed since those days? Benson has a new book, Chapman & Hall, 1871,   called Manual of Colour.

     I think it is a great improvement on the Quarto, both in size and quality. It is the size of this paper I write on.

     I have not asked you if you wish to go to sermon on Colour, for I do not think the R. I.(12) a good place to go to   of nights, even for strong men. I have, however, some tickets to spare.

     The peculiarity of our space is that of its three dimensions none is before or after another. As is x, so is y and so is  z.

     If you have 4 dimensions this becomes a puzzle,—for first, if three of them are in our space, then which three?
     Also, if we lived in space of m dimensions, but were only capable of thinking n of them, then 1st, Which n? 2d, If    so, things would happen requiring the rest to explain them, and so we should either be stultified or made wiser.

     I am quite sure that the kind of continuity which has four dimensions all co-equal is not to be discovered by merely   generalising Cartesian space equations. (I don't mean by Cartesian space that which Spinoza worked from    Extension the one essential property of matter, and Quiet the best glue to stick bodies together). I think it was   Jacob Steiner who considered the final cause of space to be the suggestion of new forms of continuity.

     I hope you will continue to trail clouds of glory after you, and tropical air, and be as it were a climate to yourself. I   am glad to see you occasionally in Nature. I shall be in London for a few days next week,—address Athenæum   Club.

     I think Strutt on sky-blue is very good. It settles Clausius's vesicular theory,

          "for, putting all his words together.
          'tis 3 blue beans in 1 blue bladder."—Mat. Prior.

     The Exp. Phys. at Cambridge is not built yet, but we are going to try. 

The desideratum is to set a Don and a Freshman to observe and register (say) the vibrations of a magnet together,     or the Don to turn a winch, and the Freshman to observe and govern him.