214 JAMES CLERK MAXWELL. [CHAP. VII. 



lect 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. 



FROM THE SAME. 



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 con- 

 stituent 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 simultane- 

 ously with equal velocities. I used black and white on 

 one of these; colours on another. Your teetotum, com- 

 bining both, I consider preferable for experiments. By the 



