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iAnfiitr’J, byM/. NewtO*.n. 88. f. 5084.

( 20 2 )

I am very sensible, that Metal Reflects leß Light than Glaß trunsoiits -and for that Inconvenience, I gave you a Remedy in my last Letter, byassigning a shallower Charge in proportion to the Aperture, than is used inother Teleseopes. But as I have found fome Mctalline Substances to bemore strongly Reflective, and to Polifh better, and be freer front Tarnilhingthan others; so 1 hope there may in time be found out some Substancc mucbfreer from these Inconveniencies than any yet lcnown.

2.. The Consulerer is pleased to reprehend me, for laying aside theThoughts of lmproving Opticks by Refractions. If he had obügcd me bya private Letter on this occasion, I would have acquainted him with mysocceß on the Trials I have made of that kind, which 1 (hall now sey havebeen leß than I sometimes expected, and perhaps than he at present hopesfor. But since he is pleased to take it for granted, that I have let this Sub-ject paß without due Examination, I (hall refer him to my formet Let-ters, by which that Conjecture will appear to be ungrounded. For, whatI seid there, was in respect of Telelcopes of the ordinary Construction,signifying, that their Improvement is not to be expected from the well figu-ring of Glasses, as Opticians have imagined 3 but I defpaired not of theirImprovement by other Constructions, which made me cautious to infertnotliing that might intimate the contrary. For although succestive Re-fractions that are all made the seme way, do necefiarily more and morsaugment the Errors of the first Refraction ; yet it seemed not impofiiblefor contrary Refractions so to correct each others Inequalities, as to maltetheir difference Regulär 3 and if that could be conveniently effected, therewould be no further difficulty. Now to this end, I cxamined, what maybe done not only by Glasses alone, but more esoecially by a Complication ofdivers lucceflive Mediums, as by two or more Glasses or Crystals with Wa-tet or some other Fluid between them 3 all which together may pejrform theOffice of one Glase, elpecially of the Object Glast, on whose Constructionthe Perfection of the Instrument chiesty depends.

To the Assertion, That Rays are leß true Reflected to a Point by aConcave, than Refracted by a Convex, I cannot assent ; nor do I under-stand, That the Focus of the latter is leß a Line than that of the former.The Truth of the contrary you will rather perceivc by the following Ta-ble, computed for soch a Reflecting Concave, and Refracling Convex, onsupposition that they have equal Apertures, and Collect Parallel Rays at ancqual Distance trom their Vertex ; which Distance being divided into x yoooparts, the Diameter of the Concave Sphere will be 60000 of those parts,and of the Convex 100003 sopposing the Sines of Inddence and Refractionto be, in round numbers, as x to 3. And this Table following flxews,hovv much the exterior Rays, at several Apertures, fäll stxort of their prin-cipal Focus.

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