Summary: combining them in various proportions by altering their angular extent. I find, however, that this method requires a little care, as a change in the angular velocity of one of the colours may alter its intensity somewhat, when the two are on the point of touching. It is very gratifying to me to find that the results I arrived at are confirmed by your observations. I believe that, personally, I have derived far more advantage from my experiments on colour, than from any other researches I have carried out, and I sincerely hope that you may have equal satisfaction in the prosecution of this and other researches. I have every reason to think that we shall meet again on some future occasion, and that we shall have an opportunity of exchanging our views on matters which are of common interest to us.