20 



A POPULAR ACCOUNT 



less refrangible. Thus if the lens be 

 exposed to a beam of violet light pro- 

 ceeding from a given object, and col- 

 lects that light to a focus at a certain 

 point, it should collect red light to a 

 focus at a more distant point, and the 

 lights of intermediate colour to points 

 between these extreme limits. 



(30.) In order to reduce the doctrine 

 to this test, Newton cast a strong red 

 light, by means of a prism, upon the 

 page of an open book in a dark room. 

 At a certain distance from the book 

 thus illuminated, he placed a double 

 convex lens, so as to give an image of 

 the book at its focus ; this image was 

 received upon a sheet of white paper 

 properly placed behind the lens. The 

 book and the lens being fixed in their 

 respective positions, the paper was 

 moved until that situation was found in 

 which the image of the page and its 

 letters were most distinctly depicted on 

 the paper. This position was of course 

 the focus to which the red light re- 

 flected from the book was collected by 

 the lens. The prism in this experiment 

 was so placed, that as the sun moved 

 in the heavens, the several coloured 

 lights of the spectrum were successively 

 cast upon the book, without disturbing 

 either its place or those of the prism 

 or the lens. The position of the book 

 was ascertained in which the letters ap- 

 peared most distinct, and it was found 

 that as the successive prismatic lights, 

 in regular order from red to violet, passed 

 over the book, the place of greatest 

 distinctness gradually approached the 

 lens, so that the full violet light required 

 for distinctness that the book should be 

 two inches and an half nearer to the 

 lens than for the red light. 



In this experiment it was necessary to 

 render the chamber extremely dark, in 

 order to prevent the pure prismatic light 

 cast upon the book from being diluted 

 by the white light which might be scat- 

 tered about the room. In proportion as 

 this adventitious light was admitted, it 

 was found that the distance between the 

 extreme foci became less. And this is 

 plainly the reason why the distance be- 

 tween the extreme foci of prismatic light 

 was found to be so much as two inches 

 and an half, while the distance for light 

 reflected from natural bodies was only 

 one inch and an half (19). For the 

 colours of natural bodies never have the 

 extreme vividness, purity, 'and splendour 

 which are obtained by the prism. 



(31.) The doctrine of the different re- 

 frangibility of light led to an obvious 



consequence respecting its inflexibility, 

 which Newton easily foresaw, and from 

 which he derived another beautiful test 

 to establish the truth of his theory. To 

 render this intelligible to those who are 

 not familiar with optical investigations, 

 we must here be permitted a short di- 

 gression. 



It will be recollected that long before 

 the time of Newton, it was known that 

 the deflection of a ray of light, in pass- 

 ing from a denser into a rarer trans- 

 parent medium, was/romthe perpendi- 

 cular. Thus, let A, Jig. 21, be air, and 

 Fig. 21. 



M ' P' 



P JV1 



G glass, and let P I be a ray incident on 

 the surface S S, which separates the air 

 from the glass. Let M M' be the per- 

 pendicular at the point of incidence I. 

 Since air is less dense or heavy than 

 glass, the ray P I, instead of persevering 

 in the direction P I, will take a direction 

 further from the perpendicular I M', 

 such as I P'. This fact was long known. 

 The law of this deflection, as discovered 

 by Snellius, has been already explained ; 

 but it may be explained under another 

 point of view, as follows : 



Round the point of incidence I, fig. 

 22, describe a circle in a perpendicular 

 Fig. 22. 



plane. Let P I be the incident ray in 

 the denser medium, and P' I the re- 

 fracted ray in the rarer medium. Draw 

 P M, P' M 7 perpendicular to the surface 

 S S'. It was found, that at whatever 

 angle P I might be incident, the propor- 

 tion of M I to M' I would be the same, 

 so long as the media on each side of 

 the surface remained unaltered. Thus, 

 suppose MI were two-thirds of M'l, 

 r 1 being incident at I, its refraction may 

 be thus found : take a distance I m! 

 from I, of which I m is two-thirds, or 

 such that I m' is equal to I m and the 



