26 



A POPULAR ACCOUNT 



we have just explained. Viewing these 

 circles thus illuminated through a prism, 

 he found that the white circle was dilated 

 into an oblong spectrum ; but that the 

 other still appeared as perfectly circular 

 as when viewed without the intervention 

 of the prism. He now placed minute 

 objects in the homogeneous light, and 

 viewing them through the prism, found 

 them distinct ; but when the same objects 

 in the sun's direct unrefracted beam 

 were similarly viewed, they became con- 

 fused and indistinct. 



law. The experiment by which this was 

 determined was as follows. 



CHAPTER IV. 



Law 



iw of refraction of homogeneous light 

 imperfection of refracting telescopes 

 Newton's reflecting telescope. 



(34.) NEWTON having now succeeded in 

 establishing the unequal refrangibility of 

 the rays which compose solar light, his 

 next step was to determine the law by 

 which each species of light was refracted. 

 It was about the year 1665, being then 

 in his 25th year, that he appears to have 

 commenced his investigations respecting 

 the composition of light ; and on the 8th 

 of February, 1672, he communicated his 

 discovery to the Royal Society, of which 

 he had just been elected a Fellow. About 

 fifty years prior to this period, Snellius 

 discovered the law of refraction, which 

 being afterwards adopted by Descartes, 

 was well known, and generally received 

 at the period to which we now allude. 

 The philosophers who observed this law 

 were not aware that all the rays of light 

 were not equally refrangible ; and New- 

 ton concluded that they adapted their 

 measures to the mean rays, or those 

 which lie in the middle of the spectrum, 

 and which therefore have an intermediate 

 refrangibility between those of the red 

 and violet, the least and most refrangible 

 rays. Hence he concludes from the 

 experiments of his predecessors, that 

 the green light of the prismatic spec- 

 trum is refracted according to the Snel- 

 lian law. 



Newton showed, by the experiment 

 which we shall now describe, that when 

 the sun's rays were all incident on the 

 prism at the same angle, the sines of the 

 refractions of the several component rays 

 always have to each other the same pro- 

 portion. It follows, therefore, that they 

 must always have the same proportions 

 to the sine of the angle of incidence, for 

 since the green ray obeys this law, and 

 they all have fixed proportions with re- 

 spect to it, they must all obey a similar 



Let RV (Jig. 29) be the spectrum 

 produced by a prism in the usual way. 

 Let another prism be placed at right 

 angles to this, as described in (24), and 

 let R' V be the new position which the 

 spectrum assumes. The distances RR', 

 V V measure upon the screen the de- 

 flections of the red and violet rays, and 

 lines parallel to these, as GG', measure 

 the deflection of the intermediate rays. 

 From the proportion of these lines, that 

 of the sines of the refractions of the 

 several kinds of rays was determined by 

 mathematical reasoning. The prism 

 placed at right angles to the first was 

 now removed, and another with a dif- 

 ferent refracting angle was substituted 

 for it, which removed the spectrum to 

 the position R r/ V". The proportion of 

 the sines of the refractions was now 

 deduced from measurement and com- 

 putation as before, and was found to be 

 unaltered. A third prism, with a dif- 

 ferent refracting angle, was used with 

 the same result. Thus it appeared that 

 the sine of refraction of each light bears 

 a fixed proportion to that of the green 

 light, while the sine of refraction of the 

 green light bears a fixed proportion to the 

 sine of incidence, by the law of Snellius ; 

 from whence it follows that in each kind 

 of light the sine of the angle of refraction 

 bears a fixed proportion to that of the 

 angle of incidence, but that this propor- 

 tion is different in light of different kinds. 



Newton gives an investigation, by 

 which he shows that this law of refrac- 

 tion may be deduced independently of 

 experiment, by mathematical reasoning, 

 from the supposition that bodies refract 

 light by acting upon its rays in directions 

 at right angles to their surfaces. We 

 shall not give here the details of this in- 

 vestigation, which also furnishes an ex- 

 planation of the fact we have already 

 stated, namely, the total reflection of the 

 light at the limiting obliquities. It is, 

 however, in the Principia that this latter 

 consequence is deduced from it. 



(35.) Having determined the proper- 



