OF NEWTON'S OPTICS. 



33 



to the different degrees of refrangibility ; 



the ray I B being after refraction re- 

 solved into I r, I o, &c. 



If a ray of light be successively trans- 

 mitted through several transparent media 

 having different refracting powers, it 

 may so happen that, on its emergence 

 from the last of these media, it shall take 

 a direction parallel to that which it had 

 when incident upon the first of them. In 

 this case the several refractions which 

 the ray suffers in passing through the 

 media, compensate and neutralise each 

 other, so as to produce, on the whole, no 

 deflection of the ray from its original 

 course. Newton observed that, under 

 these circumstances, whenever the inci- 

 dent ray was white, the refracted ray 

 was also white. But he found, on the 

 other hand, that if the refractive powers 

 of the media were not thus related, 

 and that a deflection of the incident ray 

 from its original direction finally took 

 place, a separation of the white ray into 

 its component colours was produced. 

 From these results he inferred that the 

 same succession of media, which mutu- 

 ally neutralised the refractions of any 

 one species of homogeneous light, also 

 neutralised them on all the others, so 

 that if one component part of the solar 

 beam emerged parallel to its incident 

 direction, all the others would emerge 

 with it in the same directions, thus form- 

 ing an emergent white beam. But, on 

 the other hand, that if on the whole any 

 deflection of the incident beam were 

 finally produced, such deflection would 

 be different for the different component 

 lights ; and, therefore, a decomposition 

 or dispersion would ensue. 



(4 1 .) From these facts experimentally 

 exhibited, Newton inferred, by mathema- 

 tical reasoning, the following theorems : 



I. The differences between the sines 

 of incidence and refraction, when the 

 ray passes from several different media 

 into the same medium, are to one another 

 in a given proportion. 



II. The proportion of the sines of in- 

 cidence and refraction for any one species 

 of homogeneous light from one medium 

 into another, is composed of the propor- 

 tions of these sines from the first medium 

 into any third medium, and from that 

 third medium into the second medium. 



By the first of these theorems, the re- 

 fractions of all sorts of rays from any 

 medium into air may be found, if the 

 refraction of any one sort be known. By 

 the latter, the refraction out of one me- 

 dium into another may be found, if the 



refractions of both of them into a third 

 medium be known. 



" These theorems," says Newton, 

 " being admitted into optics, there would 

 be scope enough of handling that science 

 voluminously after a new manner ; not 

 only by teaching those things which tend 

 to the perfection of vision, but also by 

 determining mathematically all kinds of 

 phenomena of colours which could be 

 produced by refractions. For to do this 

 there is nothing else requisite than to 

 find out the separations of heterogeneous 

 rays, and their various mixtures and 

 their proportions in every mixture. By 

 this way of arguing, I invented almost all 

 the phenomena described in these books, 

 besides some others less necessary to the 

 argument ; and by the successes I met 

 with in the trials, I dare promise, that 

 to him who shall argue truly, and then 

 try all things with good glasses and suf- 

 ficient circumspection, the expected event 

 will not be wanting." 



(42.) Although colour is one of the qua- 

 lities of homogeneous light, it is not, like 

 the degree of refrangibility, a test of its 

 purity or homogeneity. For compound 

 lights may be produced, the tints of 

 which will not be distinguishable from 

 those of homogeneous light. If the red 

 and yellow lights produced by a prism 

 be projected on the same white paper, 

 they will give it an orange tint, precisely 

 the same as the pure homogeneous 

 orange light, which lies between the red 

 and yellow lights in the spectrum. If 

 another white paper be illuminated by 

 this pure orange light, it will have ex- 

 actly the same appearance as to colour 

 as the paper which receives the com- 

 pound light. But if these two papers 

 thus illuminated be viewed from a dis- 

 tance through a prism, it will be found 

 that no change will take place in the ap- 

 pearance of the paper illuminated by the 

 pure orange light, while that which re- 

 ceives the compound light will be divided 

 into two images of its component colours, 

 red and yellow. In the same manner 

 any two alternate colours in the spec- 

 trum will, by their mixture, produce the 

 intermediate tint. Thus blue and yellow 

 will produce green, and so on. 



(43.) " Whiteness, and all grey colours 

 between white and black, are formed by 

 mixtures of all the colours; and the 

 whiteness of the sun's light is com- 

 pounded of all the prismatic colours 

 mixed in a due proportion." The expe- 

 riments by which Newton verified and 

 established this important proposition, 

 D 



