Letter to Henry Oldenburg, 19 March 1671

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M^r Newtons Letter to M^r Oldenb.
containing some Advertisements about his
Reflecting Telescope.

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Read Mar: 21: 71

Enter'd LB. 5. 364.

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S^r

In my last letter I gave you occasion to suspect that the instrum^nt w^ch I sent you is in some respect or other indisposed, {illeg} \or/ that the metalls are tarnished. And by yo^r letter March 16 I am fully confirmed in that opinion. For whilest I had it, it magnified represented the Moon in some parts of it as distinctly as other Telescopes usually doe w^ch magnify as much as that. {illeg}|Ye|t I very well know that that instrument hath its imperfections both in the composition of the metall & in its bei{illeg}|n|g badly cast as you may perceive by a scabrous place nere {sic} the middle of the {midd|eta|le} of the metallon y^e polished side, & also in the figured|e| of that metall neare that scabrous place. And in all those respects that instrument is capable of further improvement{.}

You seem to intimate that the proportion of 38 to 1 ho{lds} \holds/ onely for its magnifying objects at small distances. But if for such distances suppose 500 foot it magnify at that rate, by the rules of opticks it must for the greatest distance imaginable magnify more then $37\frac{3}{4}$ to 1 w^ch is so insensible a diminishing that I it may be even then put as 38 to 1.

I told you that here is anot{illeg}|h|er instrument made like the former w^ch doth very well. Yesterday I compared it w^th a six foot Telescope & found it not onely to magnif{illeg}|y| more but also more distinctly. And to day I found that at a {illeg} hundred foot distance I could reade in one of the Philosophicall Transactions p\l/aced in the sun's light at a hundred foot distance, & that at a hundred & twenty foot distance I could discern some of the words. When I made this tryall its aperture (defined {illeg} next the eye) was equalent {sic} to more then an inch and a th{illeg}|ir|d part of the object metall. This I thought good to inform you of because it may be of some use to those that shall endeavour any thing in reflect|x|ions. Fo{illeg}|r| hereby they will in some measure be enabled to judg of the goodness of their instruments{.} &|A|nd for this end you may annex these observations made w^th this last instrument, t{illeg}|o| the description of it in the Transactions of this Month. But my answer to M^r Hooks observations will not be ready for then, because I intend to annex to that answer some further explications of the Theory, w^ch I shall not have leisure to do this week or fourtnight. S^r I am in hast

Yo^r Faithfull servant.

I. Newton

March 19^th. 1671.

<36v> ^{[Editorial Note 1]}

These

For {illeg}|H|enry Oldenburg Esqꝫ
at his ho{illeg}|u|se neare the
middle of the old Pall-mail
in Westminster

London

2

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Rec. March. 20. 71.

Answ. March. 23. comm. y^e
Comet and  sub cap
Cygni from Hevel.

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i.e. if y^e \{bent}{lens}/ axis were direct {illeg}|w|ere accounted streight,

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