REFRANGIBIHTY. 



greateft rcfraAion, and were of a blue and violet colour, 

 began to be wholly rcfledled, the blue and violet lifrlit on 

 the paper, which received the rays from the fecond prifm, 

 was fenlil)ly increafed, above that of the red and yellow, 

 which was leaft refrafted ; and afterwards, when the reft of 

 the light, which was green, yellow, and red, began to be 

 wholly reflefted in the iirfl: prifm, the light of thofe colours 

 on the paper received as great an increaie as the violet and 

 blue had done before. From this it is manifeil, that the 

 beam reflefted by the bafe of the prifm, being augmented 

 firft, by the more refrangible rays, and afterwards by the 

 lefs refrangible ones, is compounded of rays difTerently re- 

 frangible. Tliis experiment is illuflrated by^yf;. 9, in wliich 

 ABC reprefents the firfl prifm, on the bale of wliich tlie 

 light falls, at M. When tliis prifm is turned about its axis, 

 according to the order of the letters ABC, the more re- 

 frangible rays, M H, emerge more and more obliquely ; 

 and at length, after their moll oblique emergence, are re- 

 flefted towards N, and going on to p, increafe the number 

 of rays N/>. By continuing the motion of the firft prifm, 

 the lefs refrangible rays, M G, are reflefted to N, and in- 

 creafe the number of rays N t. 



Since it appears from fir Tfaac Newton's experiments, 

 that different rays of light have different degrees of refrangi- 

 bility, it neceffarily follows that the rules laid down by pre- 

 ceding philofophers, concerning tlie refractive power of wa- 

 ter, glafs, &c. muff be limited to the middle kind of rays, 

 as it may be fuppofed that Kepler, Snellius, and others 

 would attend to them principally. Sir Ifaac, however, 

 proves that the fine of the incidence of every kind of light, 

 confidered apart, is to its fine of refraflion in a given ratio. 

 This he deduces both by experiment, and alfo geometri- 

 cally, from the fuppofition that bodies refraft the light 

 by afting upon its rays in lines perpendicular to their fur- 

 faces. 



Upon the whole it appears, that the blue rays are more 

 refratled than the red ones, and that there is, likewife, un- 

 equal refraction in the intermediate rays ; and upon the 

 whole it appears, that the fun's rays have not all the fame 

 refrangibility, and, confequently, are not of the fame ma- 

 ture. It is alfo obferved, that thofe rays which are moft 

 refrangible ai-e alfo moft reflexible. See the proof of this 

 under REFLEXlBlLiry. Newton's Optics, p. 22, &c. 



The difference between refrangibility and rcflexibility was 

 •irft difcovered by fir Ifaac Newton, in 167 1-2, and com- 

 municated in a letter to the Roval Society, dated Fe- 

 bruary 6, 1671-2, and publillied in the Philofophical Tranf- 

 atlions, N° 80, p. 3075, and from that time vindicated by 

 him from the objeftions of feveral authors ; particularly F. 

 Pardies, M. Mariotte, Fr. Linus, or Lin, and other 

 gentlemen at the Englifli college at Liege ; and at length 

 it was more fully laid down, illuftrated, aud confirmed, by 

 a great variety of experiments, in his excellent treatife of 

 Optics. 



But farther, as not only thofe colours of light produced 

 by refraftion in a prifm, but alfo thofe reflected from 

 opaque bodies, have their different degrees ef refrangibility 

 and reflexibility ; and as a white hght arifes from a mixture 

 of the feveral coloured rays, the fame great author con- 

 cluded all homogeneous light to have its proper colour, cor- 

 refponding to its degree of refrangibility, and not capable 

 of being changed by any refleflions, or any refraftions ; that 

 the fun's light is compofed of all the primary colours ; and 

 that all compound colours arife from a mixture of the pri- 

 mary ones, &c. 



The different degrees of refrangibility he conjeftures to 



arife from the different magnitude of the particles of which 

 the different rays confift. Thus the moft refrangible rays, 

 /. c. the red ones, he fuppofes to confift of the largeft par- 

 ticles ; tlie leaft refrangible, i. c. the violet rays, of the 

 fmalleft particles ; and the intermediate rays, yellow, green, 

 and blue, of particles of intermediate fizes. 



Having given a general view of the Newtonian theory of 

 colours, as they depend upon tlie refradlion of light, under 

 the article CoLOuit, we ftiall here add forne farther parti- 

 culars on this fubjefl. From si review of that article, and 

 of what has been above dehvered, we may infer that, as the 

 rays of light differ in refrangibility, they alfo differ in their 

 difpofition to exhibit this or that particular colour ; fo that 

 colours are not modifications of light, derived from refrac- 

 tions or refleftions of natural bodies, but original and con- 

 nate properties, which are different in different rays. More- 

 over, to the fame degree of refrangibility always belongs 

 the fame co^ur, and to the fame colour the fame degree of 

 refrangibility ; nor are the fame fpecies of colour, and degree 

 of refrangibility, that are proper to any particular kind of 

 rays, fufceptible of change by refrattion and by refleftion 

 of natural bodies, nor by any other caufe which fir Ifaac 

 Newton could obferve. Although a feeming tranfmutatioii 

 of colours may be made by a mixture of different kinds of 

 rays, yet, in fuch mixtures, the component colours them- 

 felves do not appear ; but, by their mutually allaying each 

 other, conttitute a middle colour : and, therefore, if, by 

 refraftion, the different rays be feparated, colours will 

 emerge different from that of the compofition. Thus blue 

 and yellow powders, finely mixed, appear green to the naked 

 eye ; and yet the colours of the component particles are not 

 thereby really changed, but only blended : for when they 

 are viewed with a microfcope, they ftill appear blue and 

 yellow. It appears that there are alfo two forts of colours : 

 the one original and fimple ; the other compounded of thefe. 

 The original and primary colours are red, orange, yellow, 

 green, blue, indigo, and a violet purple, and an indefinite 

 variety of intermediate gradations. The fame colours in 

 fpecie with thofe primary ones may be alfo produced by a 

 compofition : thus a mixture of yellow and blue makes 

 green ; of red and yellow, orange ; and of orange and yel- 

 lowifh-green, yellow. In general, if any two colours are 

 mixed, which, in the feries of thofe that are produced by 

 the prifm, are not too far diftant from one another, they, 

 by their mutual alloy, compofe that colour which appears 

 in the midway between them ; but thofe which are fituated 

 at too great diftance have not this effedl : e. gr. orange and 

 indigo do not produce the intermediate green, nor fcarlet 

 and green the intermediate yellow. 



The moft wonderful compofition is that of whitenefs, 

 which no one fort of rays alone can exhibit, but which is 

 always compounded ; fo that all the aforefaid primary co- 

 lours, mixed in a certain proportion, are neceflary to form 

 it. See Colour. 



Having ftiewn, in the preceding part of this article, the 

 extremes of the different degrees of refrangibility in the dif- 

 ferent kinds of hght, we fhaU now proceed to give the refult 

 of Newton's inveftigation, concerning the different degrees 

 of refrangibility of all the different kinds of light, according 

 to their feveral colours ; particularizing this part of the 

 fnbjeft with an explanation of the method which he made 

 life of to define the boundaries of each colour, in the ob- 

 long image of the lun above defcribed. In that image, 

 though there was a manifett difference of colour, not only 

 between the two extremes, but alfo in the intermediate 

 parts, yet the exatl place at which any one colour ended, 

 and another began, was far from being fuiSciently diftin- 



guiftiable. 



