REFLECTION. 



wliicli confijires with the motior: of the parts of the body, the caiife may be this ; liz. thr.t every particle of the me- 

 enter the body, arc rcfiadtc-d and tranfmittcd ; but thofe in dium has a great number of equal alternate intervals of at- 

 a contrary part of their vibration arc reflefted. traction and rcpulfion, relatively to the particles of light ; 



Add, that every ray of light, in it,, pallage through any but that thcfe intervals are of different magnitudes, accord- 

 refratting furface, is put into a certain tranfient conftitution ' ''■.■. - 



or ilate, winch, in the progrefs of the ray, returns at equal 

 intervals, and difpolcs tlie ray, at eacli return, to be eafily 

 tranfmitted through tlic next refrafting furface ; and be- 

 tween each return, to be eaiily reflcfted by it. 



Thefe alternate dilpolltions, which firlfaac Newton calls 

 /its of eafy rcjlctl'wn, and of eafy tranfm'ijjlon, he accounts for 



ing as the particles of light are of different colours. Now 

 if the thicknefs of any tranlparent medium in which the par- 

 ticles of matter are uniformly placed is fuch, that the at- 

 trafting intervals of the extreme particles, as well as the 

 repelling intervals, coincide with one another, i. e. attraftincr 

 with attrafting, and repelling with repelling, in regard to 

 __ _ ^ 3"y 0"c l^hid of rays, e. g. the red ; by the united force of 



by luppoling, that tliey are oocafioned by the vibrations of thefe extremes, (all the intermediate particles of the medium 



a fubtle fluid, in which the ray pades, which happening to 

 move fafler than the rays, when a ray is in that part of the 

 vibration which confpires with its motion, it palTes through ; 

 but when in the contrary part of the vibration, it is beat 

 back again : whence every ray is fuccefTively difpofed to be 

 eafily relieved, or eafily tranfmitted, by every vibration 

 which overtakes it. 



He alfo thought that thefe vibrations might be excited 

 by the mutual adtion and re -action of light, of bodies and 

 of this medium, at the indant of refleftion or refraftion ; fo 

 that, in faft, he fuppofes two caufes of this difpofition to 

 be refledled or tranfmitted, when rays of light arrive at any 

 neiv furface. One of them is the regular vibration of the 

 etherial medium, aficdling them through the whole of their 

 progrefs from the luminous body ; and the other the tre- 

 mulous motion, or irregular vibration of the fame medium 

 at the furfaces of bodies, occafioncd by the aftion and re- 

 aftion between light and them : for this lafl; caufe can hardly 

 be fuppofed to affeft the whole etherial medium equally, and 

 produce the reg\ilar returns of thefe fits in every ray. 



M. Bofcovieh fuppofes with Newton, that the fits of 

 refleftion or tranfmifhon affedl the rays themfelves, in the 

 whole of their padage from the luminous body, but that 

 they arife from an alteration in their form, by means of the 

 elafticity of their component parts, having been originally 

 driven from the luminous body by a force whicli aftcd more 

 ilrongly on the hinder parts than on the reil of the mafs, 

 and thereby put them into a vibratory motion. He alfo 

 fuppofes, that the intervals of the fits of eafy reflection and 

 tranfmiffion may be different in different rays, on the three 

 following accoimts. i. The rays that are differently re- 

 frangible have different velocities, at leafl after refra£tion ; 

 fo that, though the fits fliould return at equal intervals, it 

 will affedl different rays in different parts of their progrefs. 

 2. The unequal aftion between the points that compoic the 

 fame particle of light may make a difterence in their ofcilla- 

 tions, at the time of changing their medium. And laflly, 

 rays coming in diflferent inclinations to the new furface, the 

 internal motions of thefe points will be in different direftions 

 with refpeft to the furface, and coafequently the whole 

 mafs of the medium will aft upon all the points differently. 

 Bofcov. Theoria, p. 232. 



Upon the whole, fays Dr. Priefl:ley, is it not more pro- 

 bable that the rays of light are tranfmitted from the fun, 

 with an uniform difpofition to be reflected or i-efrafted, ac- 

 cording to the circumftances of the bodies on which they im- 

 pinge ; and that the tranfmiffion of fome of the rays under 

 the fame circumftances, apparently, with others that are Jledion. 

 reflefted, is owing to the minute vibrations of the fmall parts Thus 



of the furfaces of the mediums through which the rays pals ; 

 vibrations that are independent of aftion and re-aftion be- 

 tween the bodies and the particles of light at the time of 

 their impinging, though probably excited by the aftion of 

 preceding rays. As to the tranfmiffion or refleftion of cer- 

 tain kinds of light only, producing colours in thin plates, 



mutually deltroying each other's efTedls,) thefe rays will be 

 reflefted. But where the plate is of an intermediate thick- 

 nefii between this and the next thicknefs, where the attract- 

 ing intervals coincide, attrafting with attradting, and re- 

 pelling with repelling, the attradting intervals will coincide 

 with the repelling ones, and the repelling ones with the at- 

 tradting ones, and thefe mutually deflroying one another's 

 effedts, thefe rays will pafs on freely, and be tranfmitted. 

 But as the intervals of attradlion and repulfion are different 

 for differently coloured rays, the thicknefs of the plates at 

 which thefe coincidences will or will not happen, in the dif- 

 ferently coloured rays, will be different. So that it appears 

 probable, fays Dr. Prieflley, in conformity to a dodtrine 

 firfl: fuggefted to liim by Mr. Miehell, that the whole myf- 

 tery of coloured plates depends upon the attradtions and 

 rcpi'.lfions of the particles of the bodies that compofe them, 

 aftedting different rays in a different manner, according to 

 their thicknefs. Hifl. &c. of Light and Colours, p. 309, 

 &c. See Colours of thin Laminn, Sec. and Rinus cJ' 

 Co/ours. 



Sir Ifaac Newton concludes his account of the refleftion 

 of light with obferving, that if light be refledted not by 

 impinging on the folid parts of bodies, but by fome other 

 principle, it is probable that as many of its rays as impinge 

 on the folid parts of bodies are not refledted, but flifled 

 and loft in the bodies. Otherwife, he fays, we muft fup- 

 pofe two kinds of refledtion ; for fhould all the rays be 

 refledted which impinge on the internal parts of clear water 

 or cryftal, thofe iubltances would rather have a cloudy 

 colour than a clear tranfparency. To make bodies look 

 black, it is neceflary that many rays be flopped, retained, 

 and loft in them ; and it does not feem probable, that any 

 rays can be flopped and ftifled in them, which do not im- 

 pinge on their parts : and hence, he fays, we may under- 

 fland, that bodies are much more rare and porous than is 

 commonly believed. However, M. Bouguer difputes the 

 fadt of light being ftifled or loft by impinging on the folid 

 parts of bodies. See ABsoiiuiNG. 



For other fadts and obfervations relating to the fubjedt 

 of this article, fee Light. See alfo Rays of Ligl^t, and 

 Rays of Heat. 



REri.ECTiox, in Catoptrics, is the return of a ray of 

 light from the pclifhed furface of a ipeculum or mirror, 

 as driven thence by fome power refiding in it. 



The ray, thus returned, is called a reflex, or refleSed 

 ray, or a ray of reflection ; and the point of the fpeculum, 

 whence the return commences, is called the point of re- 



the ray A B [Plate I. Optics, fig. 3.) proceeding 

 from the radiant A, and ilriking on the point of the fpecu- 

 lum B, being returned thence to C, B C reprefents the 

 rejleded ray, and B the point of reflexion ; in refpedt of 

 which A B reprefents the incident ray, or ray of incidence, 

 and B the point of incidence. 



A'^gaiii, a line CG drawn from any point, as C, of the 



rc8e£ted 



