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elate was drawn gently over the fmall mirror, keeping the bodies and their interftices, which fir I. Newton has founded 

 fecondary fet of rings in view, their (hape and colour were upon the exiftencejof fits of eafy reflection and eafy tranf- 

 ibund to be always completely formed. 

 This experiment was alio repeated 



with a fmall 

 glafs, inftead of the metalline mirror put under the 

 plate. In this manner it Mill gave the fame refult, 



plain 



large 



with no 



other difference but that only fix rings could be dittm&ly 

 feen in the fecondary fet, on account of the inferior reflec- 

 tion of the fubjacent glafs. 



Our author next fhews, that coloured rings may be com- 

 pletely formed without the affiftance of any thin or thick 

 plates, either of glafs or of air. Sir I. Newton placed a 

 concave glafs mirror at double its focal length from a chart, 

 and obferved, that the refkaion of a beam of light admitted 

 into a dark room, when thrown upon this mirror, gave 

 " four or five concentric irifes or rings of colours like rain- 

 bows" (Optics, p. 265.); and he accounts for them by 

 alternate fits of eafy refleftion and eafy tranfmiflion, exerted 

 in their paflage through the glafs-plate of the concave mirror. 



Ibid. p. 277. 



The duke de Chaulnes concluded from his own experi- 

 ments of the fame phenomena, that thefe coloured rings 

 depended upon " the firft furface of the mirror, and that 

 the fecond furface, or that which reflects them after they 

 had paffed the firft, only ferved to colled them, and throw 

 them upon the pafteboard, in a quantity fufficient to make 

 them vifible." (Prieftley's Hift. &c. p. 515.) Mr. 

 Brougham, after having confidered what the two laftmen- 

 tioned authors had done, fays, " that upon the whole there 

 appears reafon to believe, that the rings are formed by the 

 firft furface out of the light, which, after reflection from the 

 fecond furface, is fcattered, and paffes on to the chart." 

 Phil. Tranf. for 1796, p. 216. 



Dr. Herfchel's experiment is as follows. He placed a 

 highly polifhed feven-feet mirror, but of metal inltead of 

 glafs, that he might not have two furfaces, at the diftance 

 of fourteen feet from a white fcreen, and through a hole 

 in the middle of it, one-tenth of an inch in diameter, he 

 admitted a beam of the fun into his dark room, fo directed 

 as to fall perpendicularly on the mirror. In this arrange- 

 ment the whole fcreen remained perfectly free from light, 

 becaufe the focus of all the rays, which came to the mirror, 

 was by reflection thrown back into the hole through which 

 they entered. After this preparation, an affiitant ftrewed 

 fome hair-powder with a puff into the beam of light, while 

 he kept his attention fixed upon the fcreen. As foon as 

 the hair-powder reached the beam of light the fcreen was 

 fuddenly covered with the mod beautiful arrangement of 

 concentric circles, difplaying all the brilliant colours of the 

 rainbow. A great variety in the fize of the rings was ob- 

 tained by making the aififtant ftrew the powder into the 

 beam at a greater diftance from the mirror ; for the rings 

 contract by an increafe of the diftance, and dilate on a 

 nearer approach of the powder. This experiment, fays our 

 author, is fo fimple, and points out the general caufes of 

 the rings which are here produced in fo plain a manner, 

 that we may confidently fay, they arife from the flection 

 of the rays of light on the particles of the floating powder, 

 modified by the curvature of the reflecting furface of the 

 mirror. From this experiment our author concludes, that 

 the principle of thin or thick plates, either of air or glafs, 

 on which the rays might alternately exert their fits of eafy 

 reflection and eafy tranfmiflion, muft be given up : and that 

 the fits themfelves of courfe cannot be fhewn to have any 

 exiftence. It will hardly be neceffary to add further, that 

 the whole theory relating to the fize of the parts of natural 



miffion, exerted differently, according to the different thick- 

 nefs of the thin plates of which he fuppofes the parts of 

 natural bodies to confift, will remain unfupported ; for if 

 thefe fits have no exiftence, the whole foundation on which 

 the theory of the fize of fuch parts is placed, will be 

 taken away, and it will be neceffary to explore another 

 bafis for a fimilar edifice. This bafis, our author conceives, 

 is to be found in the modifying power, which the two fur- 

 faces that have been proved to be effential to the formation 

 of rings, exert upon the rays of light. 



Our author having pointed out a variety of methods 

 that ferve to produce coloured concentric rings between 

 two glades of a proper figure applied to each other, and 

 having proved that only two furfaces, namely, thofe that 

 are in contact with each other, are effential to their forma- 

 tion, proceeds in the invefligation of the fubiect to (hew, 

 that prifmatic phenomena affume the fliape of rings, in con- 

 fequence of the fole ufe of Ipherical curves in producing 

 them. Our author found, by an appropriate experiment, 

 that, as fpherical curves gave circular rings, cylindrical 

 forms produce ftreaks ; that cylindrical and fpherical fur- 

 faces combined produce coloured elliptical rings ; and that 

 irregular curves produce irregular figures. Hence he in- 

 fers, that the curvature of furfaces is the caufe of the ap- 

 pearance, as well as of the fliape of the coloured pheno- 

 mena which are produced. II we can invariably predict, 

 from the nature ot the curves that are employed in an ex- 

 periment, what will be the appearance and form of the 

 colours that will be feen, it certainly muft prove the 

 efficacy of thefe curvatures in the production of fuch phe- 

 nomena. This conclulion is further confirmed by the con- 

 fideration, that coloured appearances cannot be produced 

 between the plain furfaces of two parallel pieces of glafs 

 applied to one another. 



Having proved that no more than two furfaces are effential 

 to the formation of Newton's coloured rings, and that the 

 configuration of the coloured phenomena arifes from the 

 curvature of one or both of the two effential furfaces, Dr. 

 Herfchel infers from thefe principles, that we are to diltin- 

 guilh between the production of the colours and that of 

 their configuration when produced. The caufe of the con- 

 figuration has been already explained ; and our asthor next 

 proceeds to inveftigate the production and arrangement of 

 the colours. The order of the colours is prifmatic ; that is, 

 red, orange, yellow, green, blue, indigo, and violet. Dr. 

 Herfchel's experiments for afcertaining this arrangement are 

 too numerous and various to be here recited. We (hall there- 

 fore ftate the general propofition, andfpecify the refultsof the 

 experiments by which it is eftablifhed. The general propofi- 

 tion is, that the critical feparation of the colours, which takes 

 place at certain angles of incidence, is the primary caufe 

 of the Newtonian coloured rings between optic glafles. 

 The refults of the experiments are as follow : thefe experi- 

 riments (for which we refer to Phil. Tranf. for 1809, vol. 

 xcix. pt. 2.) explain in what manner a critical feparation of 

 the colours, which takes place at certain angles of incidence, 

 is the caufe of the appearance of the blue and red bows ; 

 fince the different reflexibility of the rays of light, by which 

 Newton has accounted for the blue bow, brings on a critical 

 feparation of the blue colours, and fince alfo the different 

 intromifiibility by which the author has explained the red 

 bow, occafions an equally critical feparation of the red ones. 

 Dr. Herfchel has not only proved that all the various appear- 

 ances, which were produced by convex glafles, may be 



equally 



