TO C. J. MONRO, Esq.

                                                                    Glenlair, Dalbeattie, 6th July 1870.

     My question to the Mathematical Society bore fruit in various forms. . . . It would give my mind too great a    wrench just now to go into elliptic integrals, but I will do so when I come to revise about circular conductors. . . . I    can cut the subject short with an easy conscience, for I have no scruple about steering clear of tables of double entry, especially when, in all really useful cases, convergent series may be used with less trouble, and without any   knowledge of elliptic integrals. On this subject see a short paper on Fluid Displacement in next part of the Math. Soc. Trans., where I give a picture of the stream lines, and the distortion of a transverse line as water flows past a  cylinder.

     Mr. W. Benson, architect, 147 Albany Street, Regent Park, N.W., told me that you had been writing to Nature,    and that yours was the only rational statement in a multitudinous correspondence on colours. Mr. Benson   considers that Aristotle and I have correct views about primary colours. He has written a book, with coloured   pictures, on the science of colour, and he shows how to mix colours by means of a prism. He wants to publish an   elementary book with easy experiments, but gets small encouragement, being supposed an heretic. No other
architect in the Architect's Society believes him. This is interesting to note, as showing the chromatic condition of    architects. I made a great colour-box in 1862, and worked it in London in '62 and '64. I have about 200   equations each year, which are  reduced but not published. I have set it up here this year, and have just got it   in working order. I expect to get some more material, and work up the whole together. In particular, I want to find  any change or evidence of constancy in the eyes of myself and wife during eight years. I can exhibit the yellow   spot to all who have it,—and all have it except Col. Strange, F.R.S., my late father in-law, and my wife,—whether   they be Negroes, Jews, Parsees, Armenians, Russians, Italians, Germans, Frenchmen, Poles, etc. Professor Pole,    for instance, has it as strong as me, though he is colour-blind; Mathison, also colour-blind, being fair, had it less   strongly marked.