<22r>

Stoake: July {illeg}|8|th 1672.

Sr

I am glad to find by ye abstract of Monsr Hugenis Letter wch you transmitted to me, that he who hath done so much in Dioptricks hath been pleased to undertake the improvement of Telescopes by reflexions also, though without the desired success. For I hope the event of his next essay, if he shall think fit to attempt any thing further, will prove more happy by a little altering the manner of his proceeding. I should be glad to heare whether Mr Cock hath finished the four foot Telescope, & wt its effects are, as also what are the best effects of those of 6 or 8 inches in length wch he hath made at any time. But I know not whether I shall make any further tryalls my selfe, being desirous to prosecute \some/ another subjects.

Touching the Theory of Colours I am apt to beleive that some of the experiments may seem obscure by reason of the brevity wherewith I writ them wch should have been described more largely & explained with schemes if they had been then intended for the publick. But I see not why the aberration of the rays from the transvers of the object-glass of a Telescope should be more then about $\frac{1}{50}$ of the Glasses aperture. For suppose DF be ye Lens; CD & EF two lines parallel to its axis in or \indefinitely/ neare to wch all variety of difform rays are successively incident on two opposite parts of its Perimeter. And of those rays let <22v> DH & FG be the most refracted & DG & FH the least refracted intersecting the former in G & H. Draw GH & pr{illeg}|o|duce it both ways till at M & N it occur with CD & EF {illeg} also produced. Now since by my Principles the difference of refraction of the most difform rays is about the 24th or 25t part of their whole refraction, the angle GDH will be about a 25t part of the angle MDH & consequently the subtense GH (wch is the diameter of the least space into wch the refracted rays converge) will be about a 25t part of the subtens MH & therefore about a 49th part of ye whole line MN the diameter of the lens; or in round numbers about a fiftith part as I asserted.

This in hast Sr from

Yor Servant

I. Newton.

<22ar>
<22av>

These

To Henry Oldenburg Esq
at his house about the middle
of ye old Pall-mail in

Westminster

London

|An Extract of a letter from the same, concerning the improvemt of Telescopes by Reflexions, and his Theory of Colours.|