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The Use os thisTable will best appear by Exaniple : Suppote therefore ahalf Foot Telelcope may dillinclly Magnitie 30 times with an Inch Aperture,and it being requircd to know, wliat ought to be the Analogous Constitutionand Performance of a four Foot Tclefcope: By the lccond Column, as 100to 476 ; so are the Apertu res, as alto the numSer of times which they Mag-nitie. And contequently fince the hals Foot Tube hath an lnch Apertureand Magnitieth 30 times, a four Foot Tube proportionally tliould' have4 _34 Inches Aperture, and Magnitie 143 times. And by the third Colum»,as 100'to 168; to have their Charges : And therefore if the Diameter of theConvexity öf the EyeGlals for a halt Foot Telelcope be f of an Inch, thatfor a four Foot fhould be 4 - 5 , that is, about | of an Inch ; and to of otherLengths. But what the Event will really be we mu st wait to tee determinedby Experience. Only this I thought tit to intinuate, that they which intendto makc. tryal in other Lengths, may more readily know how.to delign theirInstruments. Thus for a four Foot Tube, fince the Aperture tliould be live-or lix Inches, there Will be requircd a piccc of Metal teven or eight InchesBroad at leatl, becaute the Figure will Icarccly be true to the Edges. Andthe t.hicknels of tlie Metal muti be proportional to the Brcadth, lest it bcridin the Grinding. The Metals being Polithcd, there may be tryals made withtevcral Eyc Glafles, to lind what Charge may with best Advantage be madeute of.

XVIII. 1. I doubt not but AI. A. will allow the Advantage of Reflexion Sotn< , o 6 j fs j; 0 „,

in the Theory to be very grear, when he flvall have informed himtelf of the °/ m. .

Different ffefrnngibility of the lhveral Rays of Light. And for the Practick ^

part, it is in tome meature manifest by the Instruments alrcady made, to,^^. 4034.what Dcgree of Vivacity and Brightnets a Mettalline Substance may be Po~lilhed. Nor is it improbable but tliat there may be ncw ways of Polifliingfound out for Metal, which will far excel thole that areyct in ute. And whena Metal is once well Polithcd, it will be a long while preterved from "Ear-nithing, if diligence be uled to keep it dry and clote, fluit up from Air:

För the principal Caule of Tarnithing leems to be, the Condenling of Moi-sture on its Polilh'd Surface, which by an Acid Spirit, where with the At-motphere is impregnated, Corrodes and Rulls it; or at least, at its Exhalingleaves it covered over with a thin Skin, confisting partly of an Earthly Sedi-ment of that moillurc, and partly of the Du st, which flying to and fro’ inthe Air had letled and adhered to it.

When there is not occalion to makc frequent ute öf the Instrument,there may be other ways to preterve the Metal for a long time , as perhaps byimmerging it in Spirit of Wine or tome other convenient Liquor. And if theychance to Tarnisti; yet their Poliflt may be rccovcred by rubbing them witha löst picee of Leathcr, or other tender iiibstance, withotit the Allistance otany fretting Powders, unletsthey happen to be Rusty: For then they mulibe ncw Polithcd.

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