FROM PROFESSOR J. D. FORBES.

                                                                        Clifton, Bristol, 4th May 1855.

 I left directions with Messrs. Neill & Co. to forward proofs of your paper, by inquiring at 18 India Street, and I   understand that they were sent out on the 1st May.

I am informed that my note to you about some of my experiments on colour has been printed in the Edinburgh    Philosophical Journal. This was by no means what I intended. . . . What I thought that you might do was to   introduce into that part of your paper where you speak of what has been done or written on the subject, mention   of the fact that as early as January 18—(I do not at the moment recollect [214] the year I stated to you) I had  used the method of rapid motion in blending colours; that I had endeavoured to obtain an equation between
certain mixed colours and pure gray; and that I had pointed out before Helmholtz, or I believe any one else, that a   mixture of yellow and blue, under these circumstances at least, does not produce green; you yourself being a  witness to what I then tried, though I was prevented from resuming the subject by ill health and some experimental occupations (conduction of heat) which I considered more imperative.

I hope you will continue to prosecute your interesting inquiries, and with an equal measure of success.

I address this to Cambridge, as I think you said you should be there this month.

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FROM PROFESSOR J. D. FORBES.
                                           .

                                                                     Clifton, Bristol, 16th May 1855.

I am much obliged by your note mentioning your intention of referring to my experiments.

You inquire how I altered the proportions of the constituent colours. My plan was, in fact, the same as yours. I   had sectors much larger than I required of each colour, making them overlap, and fixing them down by a screw at  the centre, pressing a disc of indiarubber on the discs. When I got the anomalous result of blue and yellow, I got Mr. Hay to make a disc of many alternating narrow sectors merely to see whether it might be a physiological effect from the imperfect blending of the colours.

I still think the experiment ought to be tried without motion, by winding blue and yellow threads of silk or  worsted round a card and looking at it at a good distance, or (as you proposed) by viewing it with a telescope out  of focus.

You will recollect that I had a whirling-machine (made on purpose), in which a number of discs revolved   sirnultaneously with equal velocities. I used black and white on one of these; colours on another. Your teetotum,  combining both, I consider preferable for experiments. By the  -way, I did not get the teetotum you were to  leave for me.

P.S.—I hope you have got the proof of the plate as well as of the paper. If not, write to Messes. Johnston,  engravers, 4 St. Andrew Square, Edinburgh.