London Iune 7. 73.

Sir,

Where I find you concern'd, I cannot omit giuing you notice of it, and 'tis vpon ytthat score, ytthat, though I wrote to you but the other day, when I sent you MonsrMonsieur Hugens's book de Motu Pendulorūum (wchwhich was on Thursday last by yryour carrier,) I make so much haste of writing to you again, that so I may acquaint you wthwith some particulars, come since come to my hands in a letter from the same M. Hugens, as follows in his owne words;

Paris, june 10. 1673. st. n.

Pour ce qui est des solutions de Monsieur Newton aux doutes que i'auois proposez touchant sa theorie des Couleurs, il y auroit dequoy y respondre et former encore de nouuelles difficultez; mais voyant qu'il soustient sa doctrine auec quelque chaleur, ie ne veux pas disputer. Cependant que veut dire, ie vous prie, qu'il avance, que quand mesme ie luy aurois montré que le blanc se peut composer de deux seules couleurs primitives, ie n'en pourrois pourtant rien conclurre contre luy, et cependant. il a dit pag. 3083. des Transactions, que pour composer le blanc, toutes les couleurs primitives sont necessaires.

Quant à la maniere dont il concilie l'effet des verres convexes à assembler si bien les rayons, auec ce qu'il establit touchant la differente refrangibilité; I'en suis satisfait: mais aussi doit il avouër, que cette aberration des rayons ne nuit donc pas tant aux voiz verres, qu'il semble auoir voulu faire à croire, quand il a proposé les miroirs concaves comme la seule esperance de perfectioner les Telescopes. Son Invention assuremtassurement estoit tres-belle, mais, à ce que i'ay pû conoitre par l'experience, le defaut dela matiere la rend presque aussi impossible d'executer, que la difficulté dela forme repugne aux Hyperboles de M. des Cartes: de sorte qu'à mon avis il en faudra deneurer à nos verres spheriques, auxquels nous auons desia tant d'obligaonobligation, et qui peuvent recevoir encor plus grande perfection tant par l'augmentation dela longueur des lunettes, que par la correction dela matiere du verre mesme.

So far He: wchwhich I thought fit to hasten to you, ytthat you might, if you then pleas'd, take notice of it to him, when you send him yryour thanks for his book.

The words of yryour Answer of April. 3. 73, wchwhich he thinks doe clash wthwith those in yethe Transactions, are these;

if therefore, M. Hugens would conclude any thing, he must show, how white may be produced out of two vncompoundded colors; wchwhich when he hath done, I will further tell him, why he can conclude nothing from that?

This, I suppose, was not said by you in contradiction to ytthat paragraph of p. 3083. in Transactions, but wthwith refe[33]rence to yethe main import of yryour Theory, viz. ytthat ytthat would not be overthrown by it, though perhaps ytthat part concerning yethe composition of whiteness should fall.

But you I leaue it to yrselfyourself, to satisfy M. Hugens, exspecting yryour notice of having receiued this from

Sir

yryour humble and faithful servant

Oldenburg

To his honored Friend Mr Isaac Newton professor of the mathematiques, at his Chamber in Trinity-Colledge in

Cambridge

2