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es Refiexton toOftic\lpfai~mtnti i byMr. Newton,n, .So. f. 307?.

( > 9 6 )

XVII. 1. When I had found, That Light consißs of Rjiys dijferemly 1{e-frangible, I lest oft' my Glals-works, for I law that the Perfection of Tele-skopes was hitherto limited, not so much for want of Glasses truly figurcdaccording to the Presoriptions of Optick Authors, (which ali Men have hi-therto imagincd) as bccaufe that Light it seif is a Heterogeneous mixture ofDifferenti} Rcfrangible fttys. So that, were a Glass so exactly figured, as tocollect any one fort of Rays itito one Point, it could not collect those also in-to the fame Point, which having the fame Incidence upon the fame Mediumare apt to fufter a different Refraction. Na)', I wondred, That feeing- theDifterence of Refrangibiiity was so great, as I found it, Telefcopcs fhouldarrive to that Perfection they are now at. For, mcafuring the Refractionain one of my Prifms, I found, that fuppofing the common Sine of Incidenceupon one. of its Plains was 44 Parts, the Sine of Refraction of the utmoilRays on the red end of the Colours, made out of the Glafs into the Air,would be 68 parts, and the Sine of Refraction of the utmoft Rays 011 theother end, 69 parts : so that the Difterence is about a a 41h or ayth part ofthfc whole Refraction. And consccjuently, the Object Glafs of any Telescopecannot Collect all the Rays, which come front one Point of an Object, soas to make them convene at its Focus in lefs room than in a Circular (pace,whofe Diameter is the yoth part of the.Diameter of its Aperture; which isan Irregularity, forne hundreds of times, greater than aCircularly figured Lens,of so finali a Section as the Object Glasses of Jong Teleskopes are, would causoby the unfitneß of its Figure, were Light Uniform.

This made me take Reflexions into Consideratioh; and finding them Re»gular, so that the Angle of Reflexion of all sorts of Rays was equal to. theiuAngle of Irtcidence, I understood, that by their Mediation Optick ..Instrutments might be brought to any degree of Perfection imaginable, providcd nReflecting Substance could be found, which would. Polisti as finely as Glafs»and Reflect as much Light as Qlasi Trans mits, and die Art of communicatingto it a Parabolick Figure be .also attained. But there soemed very great Dit-siculties, and I have almost thought them Infupcrable, when I farther. con-.sidered, that every Irregularity in a Reflecting Superficies rnakes the Rays stray.y on 6 times more out of their due Courso, than the like. Irregularities in aRefracting one: So that a much greater Curioftty would be. herexequiiite,:than in figuring Glasses for Refraction.

Amidst these Thoughts, I was forced fv.om Gamkridgc , Anno 1 666. by the inter-,vening Plague, and it was more than two years before I proceeded further. Butthen having thought on a tender way of Polifhing, proper for Metal, where-,by as I imagined the Figure also would be corrected .to the lass, I beganto try what might be effected in this kind, and by degrees so far perfectcd a:v.Instrument (in the. Essential parts. of it like that I fent to London ) by which Icould diseem Jupiters 4 Concomitatits, and fhewed them divers times to,two others of my Acquairitance. I could also difeern the. Moon-like: Phaseof Venus, but not very dSstirictly, nor without some niceneß in dispo fing jhcInstrument.