An Extract of a Letter lately written by an ingenious person from Paris, containing some Considerations upon Mr. Newtons Doctrine of Colors, as also upon the effects of the different Refractions of the Rays in Telescopical Glasses.

I Have seen, how Mr. Newton endeavours to maintain his new Theory concerning Colours. Me thinks, that the most important Objection, which is made against him by way of Quære, is that, Whether there be more than two sorts of Colours. For my part, I believe, that an Hypothesis, that should explain mechanically and by the nature of motion the Colours JellowYellow and Blew, would be sufficient for all the rest, in regard that those others, being only more deeply charged (as appears by the Prismes of Mr. Hook,) do produce the dark or deep-Red and Blew; and that of these four all the other colors may be compounded. Neither do I see, why Mr. Newton doth not content himself with the two Colors, Yellow and Blew; for it will be much more easy to find an Hypothesis by Motion, that may explicate these two differences, than for so many diversities as there are of othersother Colors. And till he hath found this Hypothesis, he hath not taught us, what it is wherein consists the nature and difference of Colours, but only this accident (which certainly is very considerable,) of their different Refrangibility.

As for the composition of White made by all the Colors together, it may possibly be, that Yellow and Blew might also be sufficient for that: Which is worth while to try; and it may be done by the Experiment, which Mr. Newton proposeth, by receiving against a wall of a darkn'd room the Colours of the Prisme, and to cast their reflected light upon white paper. Here you must hinder the Colors of the extremeties, viz. the Red and Purple, from striking against the wall, and leave only the intermediate Colors; yellow, green and blew, to see, whether the light of these alone would not make the paper appear white, as well as when they all give light. I even doubt, whether the lightest place of the yellow color may not all alone produce that effect, and I mean to try it at the first conveniency; for this thought never came into my mind but just now(6087) now. Mean time you may see, that if these Experiments do succeed, it can no more be said, that all the Colors are neccessary to compound White, and that 'tis very probable, that all the rest are nothing but degrees of Yellow and Blew, more or less charged.

Lastly, touching the Effect of the different Refractions of the Rays in Telescopical Glasses, 'tis certain, that Experience agrees not with what Mr. Newton holds. For to consider only a picture, which is made by an object-glass of 12 feet in a dark room, we see, it is too distinct and too well defined to be produced by rayes, that should stray the 50th. part* Compare herewith what Mr. Newton saith in Numb. 80. of these Tracts, pag. 3079. of the Aperture. So that, (as I believe I have told you heretofore) the difference of the Refrangibility doth not, it may be, alwayes follow the same proportion in the great and small inclinations of the Rayes upon the surface of the Glass.