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by a denfer. From the preceding phenomena it is an ob- 

 vious deduction, that the tranfparent parts of bodies, ac- 

 cording to their feveral fizes, reflect rays of one colour, and 

 trantmit thofe of another ; on the fame account that thin 

 plates, or bubbles, refleft or tranfmit thofe rays ; and this 

 Newton fuppofed to be the reafon of all their colours. 

 Hence alfo he has inferred, that the fixe of thofe component 

 parts of natural bodies that afted the light, may be conjee- 

 tured by their colours. See Colour and Reflection. 



Sir Ifaac Newton, purfuing his difcoveries concerning the 

 colours of thin fubftances, found that the fame were alfo 

 produced by plates of a confiderable thicknefs, divilible into 

 leffer thickneffes. The rings formed in both cafes have the 

 fame origin, with this difference, that thofe of the thin 

 plates are made by the alternate reflections and tranfmiffions 

 of the rays at the'fecond furface of the plate, after one paf- 

 fage through it ; but that, in the cafe of a glafs fpeculum, 

 concave on one fide, and convex on the other, and quick- 

 filvered over on the convex fide, the rays go through the 

 plate, and return before they are alternately reflected and 

 tranfmitted. Newton's Optics, p. t6o, Sec. or Newton's 

 Opera, Horfley's edit. vol. iv. p. 121, Sec. p. 184, &c. 



The abbe Mazeas, in his experiments on the rings of co- 

 lours that appear in thin plates, has difcovered feveral im- 

 portant circumftances attending them, which were over- 

 looked by the fagacious Newton, and which tend to invali- 

 date his theory for explaining them. In rubbing the flat 

 fide of an object-glafs againit another piece of flat and 

 fmooth glafs, he found that they adhered very firmly to- 

 gether after this friftion, and that the fame colours were 

 exhibited between thefe plane glaffes, which Newton had 

 obferved between the convex object-glafs of a telefcope, 

 and another that was plane ; and that the colours were in 

 proportion to their adhefion. When the fin-faces of pieces 

 of glafs, that are tranfparent and well polithed, are equally 

 preffed, a refiflance will be perceived ; and wherever this is 

 felt, two or three very fine curve lines will be difcovered, 

 fome of a pale red, and others of a faint green. If the 

 friction be continued, the red and green lines incrcafe in 

 number at the place of contact ; the colours being fometimes 

 mixed without any order, and fometimes diipofed in a regu- 

 lar manner ; in which cafe the coloured lines are generally 

 concentric circles, or ovals, more or lefs elongated, as the 

 furfaces are more or lefs united. When the colours are 

 formed, the glaffes adhere with confiderable force ; but if 

 the glaffes be feparated fuddenly, the colours will appear 

 immediately upon their being put together, without the leaft 

 friction. Beginning with the flighted touch, and increafing 

 the preffure by infenfible degrees, there firft appears an oval 

 plate of a faint red, and in the centre of it a fpot of light 

 green, which enlarges by the preffure, and becomes a green 

 oval, with a red fpot in the centre ; and this enlarging, in 

 its turn, difcovers a green fpot in its centre. Thus the red 

 and green fucceed one another in turns, affuming different 

 (hades, and having other colours mixed with them. The 

 greateit difference between thefe colours exhibited between 

 plane furfaces, and thofe by curve ones, is, that, in the 

 former cafe, preffure alone will not produce them, except 

 in the cafe above-mentioned. In rubbing together two 

 prifms, with very fmall refracting angles, which were joined 

 fo as to form a parallelepiped, the colours appeared with a 

 furprifing luftre at the places of contact, and differently 

 coloured ovals appeared. In the centre there was a black 

 fpot, bordered by a deep purple ; next to this appeared 

 violet, blue, orange, red tinged with purple, light green, 

 and faint purple. The other rings appeared to the naked 

 *ye to confift of nothing but faint reds and greens. When 



3 



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thefe coloured glaffes were fufpended over the flame of a 

 candle, the colours difappeared fuddenly, though they (till 

 adhered ; but being fuffered to cool, the colours returned 

 to their former places, in the fame order as before. At firft 

 the abbe Mazeas had no doubt but that thefe colours were 

 owing to a thin plate of air between the glaffes, to which 

 Newton has afcribed them ; but the remarkable differences 

 in the circumftances attending thofe produced by the flat 

 plates, and thofe produced by the objeft-glalles of Newton, 

 convinced him that the air was not the caufe of this appear- 

 ance. The colours of the fiat plates vanilhed at the ap- 

 proach of flame, but thofe of the object-glaffes did not. 

 Nor was this difference owing to the plane glaffes being lefs 

 comprefled than the convex ones ; for though the former 

 were compreffed ever fo much by a pair of forceps, it did 

 not in the leaft hinder the effect of the flame. Afterwards 

 he put both the plane glaffes and the convex ones into the 

 receiver of an air-pump, fufpending the former by a thread, 

 and keeping the latter comprefled by two firings ; but he 

 obferved no change in the colours of either of them, in the 

 moil perfect vacuum that he could make. Sufpe&ing itill 

 that the air adhered to the furface of the glaffes, fo as not 

 to be feparated from them by the force of the pump, he had 

 recourfs to other experiments, which rendered it Itill more 

 improbable that the air fhould be the caufe of thefe colours. 

 Having laid the coloured plates, after warming them gradu- 

 ally, on burning coals ; and thus, when they were ncariv 

 red, rubbing them together, he obferved the fame coloured 

 circles and ovals as before. When he ceafed to prefs upon 

 them, the colours feemed to vanilh ; but they returned, as 

 he renewed the friction. In order to determine whether the 

 colours were owing to the thicknefs of fome matter inter- 

 pofed between the glaffes, he rubbed them together with 

 fuet and other foft fubftances between them ; yet his endea- 

 vours to produce the colours had no effect. However, by 

 continuing the friction with fome degrees of violence, he ob- 

 ferved, that a candle appeared through them encompailed 

 with two or three concentric greens, and with a lively red 

 inclining to yellow, and a green like that of an emerald ; 

 and at length the rings aflumed the colours of blue, yellow, 

 and violet. The abbe was confirmed in his opinion that 

 there muft be fome error in Newton's hypothefis, by con- 

 fidering that, according to his meafures, the colours of the 

 plates varied with the difference of a millionth part of an 

 inch ; whereas he was fatisfied that there muft have been 

 much greater differences in the diftance between his glaffes, 

 when the colours remained unchanged. From other experi- 

 ments he concluded, that the plate of water introduced be- 

 tween the glaffes was not the caufe of their colours, as 

 Newton apprehended ; and that the coloured rings could 

 not be owing to the compreffion of the glaffes. After all 

 he adds, that the theory of light, thus reflected from thin 

 plates, is too delicate a fubject to be completely afcertained 

 by a fmall number of obfervations. Berlin Mem. for 1752, 

 or Memoirs Prefentes, vol. ii. p. 28 — 43. 



For M. du Tour's experiments and obfervations, fee 

 Mem. Pref. vol. iv. p. 288. 



M. Mufchcnbroeck is alfo of opinion, that the colours of 

 thin plates do not depend upon the air ; but as to the caufe 

 he is not fatisfied. lntrod. ad Phil. Nat. vol. ii. p. 738. 

 See on this fubject Prieftley's Hift. of Light, &c. per. 6. 

 fedt. 5. p. 498, &c. 



Sir Ifaac Newton's explanation of the appearance of the 

 above-mentioned coloured concentric rings, who accounts 

 for the production of them by afcribing to the rays of 

 light certain fits of eafy reflection and eafy . tranfmifficn 

 alternately returning and taking place with each ray at 



