TO C. J. MONRO, Esq.

                                                        8 Palace Gardens Terrace,
                                                        London W, 18th February 1862.
                                                        (Recd. 3d March.)

     I got your letter in Scotland, whither we had gone for the Christmas holidays. I have been brewing Platonic suds,    but failed, owing I suppose to a too low temperature. I had not read Plateau's recipe then. Some of the bubbles  on the surface lasted a fortnight in the air, but they were scummy and scaly and inelastic. I shall take more care   next time. Elliot of the Strand is going to produce colour-tops, with papers from De La Rue, and directions    for use by me; and so I shall be put in competition with the brass Blondin and the Top on the top of the Top.
. . . With regard to Britomart's nurse—I have not Spenser here, but I think Spenser was not a magician    himself, and got all his black art out of romances and not out of the professional treatises,—the notions to be   brought out were:—1st, The unweaving any web in which B. had been caught; 2d, Doing so in witch-like fashion;   3d, Not like a wicked witch, but like a well-intentioned nurse, unused to the art, and therefore blunderingly. She   believes in the number three and in contrariety, and therefore says everything thrice and does everything thrice,
     saying inversions of sentences, and doing reversions of her revolutions, which are described in similar language   The revolutions begin by + 3 (2) against the visible motion of the sun, then by a revolution — 6 or she returns  all contrary and unweaves the first. Then she goes round + 6, to make the final result contrary to the natural    revolution, and to make a complete triad. Withershins is, I believe, equivalent to wider die Sonne in High Dutch,   which I am not aware is a modern or ancient idiom in that language, but it may be one in a cognate language. If the
"phamplets" have not turned up in Madeira yet, let me know, that I may "replace" them.

 I suppose in your equations, when the numbers do not amount to unity, Black has been present.

                           ·841 Brunsw. ·G + ·159 W = ·200 V + ·423 U + ·377 Black,

     That is, green a little palish and dark mauve, your last equation by the young eyes. It is something like a    colour-blind eqn., but all those I know say 100 Brunswick G = 100 Vermillion, so that this person sees the green    darker than the Vermillion, or in other words sees much more of the second side of the equation than a   colour-blind person would. But in twilight U comes out strong, while G does not; so that I think the apparent    equality arises from suppression of all colours but blue (in U and W) in the twilight, so that you may write—

                                 ·841 Black + ·159 W = ·577 Black + ·423 U.

     There is no use going to the 3rd. place of decimals, unless you spend a good while on each observation, and have    first-rate  eyes. But if you can get observations to be consistent to the 3rd. place of decimals, glory therein,   and let me know what the human eye can do.

     Donkin gave me tea in Oxford, July 1, 1860.

     I find that my belief in the reality of State affairs is no greater in London than in Aberdeen, though I can see the   clock at Westminster on a clear day. If I went and saw the parks of artillery at Woolwich, and the Consols going   up and down in the city, and the Tuscarora and Mr. Mason. I would know what like they were, but otherwise a    printed statement is more easily appropriated than experience is acquired by being near where things are being     transacted.

     I am getting a large box made for mixture of colours. A beam of sunlight is to be divided into colours by a prism,    certain colours selected by a screen with slits. These gathered by a lens, and restored to the form of a beam by   another prism, and then viewed by the eye directly. I expect great difficulties in getting everything right adjusted,  but when that is done I shall be able to vary the intensity of the colours to a great extent, and to have them far  purer than by any arrangement in which white light is allowed to fall on the final prism.

     I am also planning an instrument for measuring electrical effects through different media, and comparing those    media with air. A and B are two equal metal discs, capable of motion towards each other by fine screws; D is a   metal disc suspended between them by a spring, C; E is a piece of glass, sulphur, vulcanite, gutta-percha, etc. A   and B are then connected with a source of + electricity, and D with - electricity. If everything was symmetrical, D    would be attracted both ways, and would be in unstable equilibrium, but this is rendered stable by the elasticity of
the spring C. To find the effect of the plate E, you work A further or nearer till there is no motion of D consequent     on electrification. Then the plate of air between A and D is electrically equivalent to the two plates of air and one    of glass (say) between D and B, whence we deduce the coefft. for E.