REFRANGIBILITY. 



twoen 77 and 78 be divided, as tlic line GM h divided by 

 thofe intervals, there will be 77, 771, 77r!> 77!> 774> 77?) 

 77-J, 78, for the fines of refraflion of thofe rays out of 

 glafs into air, their common fine of incidence being 50. So 

 then the fines ot the incidences r,f all the red-making rays 

 out of glafs into air were to the fines of their refraftions not 

 greater than 50 to 77, nor lefs than 50 to 77; ; but they 

 varied from one another according to all their intermediate 

 proportions, and fo of the other colours. 



Having demon llrated that the light of the fun confifts of 

 a certain proportion of differently coloured light, our author 

 proves, by another feries of experiments, what has been al- 

 ready demon llraled by a fingle one mentioned above, that 

 when a beam of light has been divided into its component 

 parts, if they be again mixed, they will produce white ; or 

 if any one of them be intercepted, the image will appear 

 tinged, and in a difterent manner, according to the different 

 colours that are thereby prevented from mixing with the 

 reft : and to complete the whole, he obfcrves, that inter- 

 cepting all the colours that compofe the white image except 

 one, and thereby making it to exhibit the appearance of all 

 the colours in order ; yet, if he made this lucceifion of all 

 the colours very quick, the appearance was always white, 

 though it was demonftrable that only one colour took place 

 at any one time ; and he juilly obfcrves, that if each of thefe 

 colours in fuccefiion give the idea of whitcnefs, much more 

 will they produce that effeft, when they are fo intimately 

 mixed as they are in a natural fun-beam. 



Not content with compofing whitenefs from the fepa- 

 rately coloured rays of the fun, he attempted, and fucceeded 

 in his attempt, to do the fame with natural coloured bodies, 

 obfcrving the fame proportions of the refpedtive colours 

 that he had found in the folar image. The coloured pow- 

 ders which he made ufe of at firll produced only a kind of 

 grey ; but this was in faft a dull white, or whitenefs mixed 

 with fhade : for when he contrived to throw a very ftrong 

 light upon it, it became intenfely white, fo that a friend of 

 his, who happened to call upon him while he was bufy 

 about thefe curious experiments, and who knew nothing of 

 what he had been doing, pronounced that the powders he 

 had been mixing, when thus illuminated, made a better 

 white than fome very fine white paper, with which he was 

 comparing it. 



As the preceding proportion of all the prifmatic colours 

 makes a white, it is evident that when they are mixed in 

 different proportions, or when only a few of them are ufed, 

 they will make different colours j and our philofopher has 

 given us the following ingenious method of knowing, in a 

 mixture of primary colours, the quantity and quality of each 

 being given, the colour of the compound. With the centre 

 O, [Jig. 14.) and the radius O D, defcribe a circle A D F, 

 and divide the circumference into feven parts, proportional 

 to the feven mufical tones or intervals contained in an oc- 

 tave, that is in proportion to the numbers 4, -;-•;, -,\;, 4, 

 ^, -j^, ^, Let the firft, D E, reprefent a red colour, the 

 fecond, E F, orange, and fo of the reft ; and let all thefe 

 ■colours be fuppofed to pafs gradually into one another. 

 Let p be the centre of gravity of the arc D E, and g, 1; s, 

 t, u, x, be the centres of gravity of the other arcs ; and 

 about thofe centres let circles, proportional to the number 

 of rays of each colour in the given mixture, be defcribed. 

 Then find a common centre of gravity of all thofe circles ; 

 and if a line be drawn from the centre of the circle O, 

 through this point, as iuppofe at z, the point Y, in which 

 it terminates at the circumference, will ikew the colour that 

 arifes from the mixture ; and the hue O Z will be propor- 

 tioned to the fuUnefs or intenfencfs of the colour, the centre 



O reprcfenting ptrfett white. But if only two of the pri^ 

 mary colours, which in this circle are oppofite to one 

 another, be mixed in an equal proportion, tlie point Z will 

 fall upon the centre O, and yet the colour compounded of 

 thofe two will not be perfectly white, but fome faint anony- 

 mous colour ; for he could never, by mixing only two pri- 

 mary colours, produce a perfeft white. Whether it might 

 be the refult of three taken at equal diftances in the circum- 

 ference he could not tell ; but he did not much qucftion, 

 but that four or five of them would be fufiieient. Thefe, 

 however, as he obferves, are curiolities of little or no mo- 

 ment to the underftanding of the phenomena of nature, 

 fince, in all natural whites, there is a mixture of all kinds 

 of rays. Newton's Optics. Prieftley's Hift. of Light and 

 Colours. 



^For the method of correcting the effect of the different 

 refrangibility of the rays of light in glaffes, fee Aniiiu'.A- 

 Tio.v and Telescope ; fee alfo Dispeksion 0/ Light. 



Refrangibility 0/ Radiant Heat. See Rays of Heat. 

 Radiant heat, as well as light, is not only refrangible, but 

 it is alfo fubjeft to the laws of the difperfion, arifing from 

 its different refrangibility. The prifm refrafts radiant heat, 

 fo as to feparate that which is lefs efficacious from that which 

 is more fo. The whole quantity of radiant heat contained 

 in a fun beam, if this different refrangibility did not exift, 

 muft inevitably fall uniformly on a fpace equal to the area of 

 the prifm ; and if radiant heat were not refrangible at all, it 

 would fall upon an equal fpace, in the place where the 

 I'hadow of the prifm, when covered, may be feen. But 

 neither of thefe events taking place, it is evident tiiat the 

 radiant heat is fubjeft to the laws of refraftion, and alfo to 

 thofe of the different refrangibility of light. Whence Dr. 

 Herfchel is led to furmife, that radiant heat con fids of 

 particles of light of a certain range of momenta, which 

 range may extend a little farther, on eacl^fide of refrangi- 

 bility, than that of light. ( See Rays of Heat. ) Dr. Herf- 

 chel having found that two degrees of heat were obtained 

 from that part of the prifmatic fpeftrum which contained the 

 violet rays, while the full red colour, on the oppofite fide, 

 gave no lefs than feven degrees, infers from thefe fafls the 

 different refrangibility of the rays which occafion heat, as 

 clearly and certainly as it is concluded that the refrangibility 

 of light is afcertained by the difperfion and variety of the 

 colours. But he proceeds farther, and obferves, that the 

 rays of heat are of a much more extenfive refrangibility than 

 thofe of light. In order to make this appear, he deli- 

 neates a fpeftrum of light, by affuming a line of a certain 

 length ; and, dividing it into feven parts, according to the 

 dimenfions affigned to the feven colours by fir Ifaac Newton, 

 in the fourth figure of the fecond part of his Optics, 

 reprefents the illuminating power of which each colour 

 is poffefled, by an ordinate drawn to that line. And here, 

 as the abfolute length of the ordinates is arbitrary, provided 

 they be proportional to each other, he affumes the length 

 of that which is to exprefs the maximum, equal to -^4 of the 

 whole line. 



Thus, let G Q {fg. 15.) reprefent the line that contains the 

 arrangement of the colours, from the red to the violet. 

 Then,ereftingon the confines of the yellow and green the line 

 L R ;:z 4-J of G Q, it will reprefent the power of illumi- 

 nation of the rays in that place. For, by experiments al- 

 ready delivered, we have ihewn that the maximum of illumi- 

 nation is in the brighteft yellow or paleft green rays. From 

 the fame experiments we colleft, that the illuminations' of 

 yellow and green arc equal to each other, and not much in- 

 ferior to the maximum ;"thi3 gives us the ordinates K and M. 

 Then, by the reft of the fame experiments, we obtain alfo 



the 



