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other ; and the Length os the Figure being not Parallel but Transverse ro-the Axis os the Prisin. Aster this manner I used to try the Experiment ;and it will not socceed well is the Day be not Ciear, and the Prisin placedClosc to the Hole, or so near at least, that all the Sun’s Light that comesfrom the Hole may paß through the Prisin also, so as to appear in a Round.Form, if intercepted by a Paper immediately aster it has pasted the Prisrn.

When Mr. Line has tryed this, I could wish he would proceed a little fur-ther, to try that which I called the Experimentum Crucis- For when he hastryed them (which by his denying them, I know he has not done yet as theyshould be tryed) I presome he will rest fatisfied. It may be tryed (thoughnot so perfectly) even without darkning a Room, or the Expence of anymore Time than half. a quarter of an Hour.

5. Mr. Linus (now deceased) tryed tbe Experiment again and again, and'T he Experiment divers on purpose to scc ir, nor ever made difficulty to (hew it to any

one, who either by chance came to his Chamber as he was doing it, orMr.Gascoigne, shewed the least desire to see the lame ; so that sor point of Experience,n, 121./>.503. jyj r Nmton cannot be more confident on his fide, than wc are here on theother ; who are fully perswaded, that, unleß thediverfity ofplacing the Prism,or the bigneß of the Hole, or some other soch circumstance, be the cause ofthe difference betwixt them,. Mr. Newton s Experiment will hardly stand-

6. By Mt. Gascoigne’ s Letter one might fidpect, that. Mr. Linus tryed theExperiment some other way than I did ; and therefore I. shall cxpect, till hisFriends have tryed it according to my late Directrons. In which tryal it mayposiibly be a further guidance to them, to acquaint them, that the Prism castsfrom it several Images. One is, that Oblong one of Colours which I mean;and this is made by two Refractions only. Another there is made by two.Refractions and an Interveni'ng Reftcction; and this is Round and Colourlels,if the Angles of the Prisrn be exactly equal; but if the Angles at the Reste-cting Base be not Equal, it will be Colour’d, and that so much the more, byhow much Unequaller the Angles are, but yet not much Unround, unleß theAngles be very Unequal. A Third Image there is, made by one fingleRestection, and this is always Round. and Colourleß. The only danger isin mistaking the Sccond sor the First. But they are distinguishable not onlyby the Length and Lively Colours of the first, but by its disterent motiontoo: For, whilst the Prism is turned continually the fame way about its Ax-is, the Sccond and Third move swiftly, and go always on the fame way tillthey ditäppear; but the First moves flow, and grows continually flowcr tillit be Stationary, and then turns back again, and goes back faster and fasterztill it vanilh in the Place where it began to appear.

If without Darkning their Room they hold the Prisin at theil- Windowin the Sun’s open Light, in a such a posture that its Axis be perpendicular tothe Sun-Beams, and then turn it about its Axis, they cannot miß of seeingthe sirst Image; which having found, they may double upa Paper once ortwicc, and make a round. Hole in the middle of it, about or 4 of an inchbroad, and hold :the. Paper immediately before the Prisin, that the Sun may(Eine on the Prisin through that Hole; and the Prisin being stayed,- and

. heH

narg.

\Anfieer’i by

Mr. Newton.&id. n. 123.-f>-55^