18 



A POPULAR ACCOUNT 



reflected from the surfaces of natural 

 bodies were differently refrangible ; and 

 subsequently he shewed that the colours 

 produced by the refraction of a sunbeam 

 by a prism were also differently re- 

 frangible, and that the colours which, 

 reflected from natural bodies, were most 

 refrangible were also the colours most 

 refrangible in the refracted solar light. 

 But in order, as it were, to identify the 

 two experiments on natural colours and 

 coloured light, he instituted the following 

 experiment. 



(27.) By means of two prisms, as de- 

 scribed in (25), he projected two spec- 

 trums on the wall, so as to be placed 

 end to end in the same direction, and 

 so that the violet end of the one joined 

 the red end of the other. At some dis- 

 tance from the wall he placed a slender 

 piece of white paper, with straight and 

 parallel edges, and so arranged that the 

 red light R,Jrg. 18, of the one spectrum 



Fig. 18. 



ILOTGBIV 



should illumine one half of 1he paper, 

 while the violet light, V, of the other 

 spectrum illuminated the other half, 

 The paper thus appeared of two colours, 

 red and violet, similar to the painted 

 paper used in the experiment described 

 in (18). The remaining lights of each 

 spectrum passing beyond the paper fell 

 upon the wall, which was hung with 

 black, in order that light reflected from 

 it might not disturb the experiment. It 

 is evident that this arrangement was 

 such as to place the coloured light pro- 

 duced by the refraction of the prism 

 exactly under the same circumstances 

 as the light reflected by the colours of 

 natural bodies in the experiment already 

 alluded to. Accordingly Newton viewed 

 the illuminated paper through a prism 

 held parallel to it, as in that experiment, 

 and found exactly the same result, vis:. 

 that the violet half was separated from 

 the red by a greater refraction, so that 

 the parts of the paper, instead of forming 

 one straight band, were now separated 

 from one another, but placed in parallel 

 directions. Instead of using a band of 

 paper, he sometimes used a white thread, 

 one half of which he placed in the violet, 

 and the other in the red light, and ob- 

 served the same effect, the thread 

 appearing to be broken, and one half of 

 it moved out of its place, but parallel to 

 its former position. 



In this experiment, by turning one of 

 the prisms upon its axis, he was enabled 

 to illuminate one half of the thread 

 successively with the violet, indigo, 

 blue, green, and the other prismatic co- 

 lours, while the other prism, main- 

 taining its position constantly illuminated 

 the other half of the thread with red 

 light. Upon viewing these phenomena 

 successively with a third prism, he found 

 that in each case the parts of the thread 

 illuminated with lights of different co- 

 lours were separated, but the separation 

 was greatest between the red and violet, 

 less between the red and indigo, still less 

 between the red and blue, and so on, 

 being very small between the red and 

 orange. But when both parts of the 

 thread were illuminated with the reds of 

 the two spectra, the thread appeared no 

 longer broken. It is scarcely necessary 

 to observe, that all these phenomena 

 were such as must have been easily 

 foreseen from the supposed unequal re- 

 frangibility of differently coloured lights. 

 Newton might have carried this expe- 

 riment further, and probably he did so, 

 although he has not particularly men- 

 tioned it. He might have thrown on 

 the thread every possible distinct pair 

 of the prismatic colours, by moving both 

 prisms on their axis, and the result 

 would be that the apparent separation 

 of the parts of the thread, when viewed 

 through the prism, would have been 

 great in proportion to the distance be- 

 tween the two colours in the spectrum. 



(28.) All those experiments instituted 

 by Newton, in which the refracted light 

 was received upon a screen, were re- 

 peated with the same success, the light 

 being admitted immediately to the eye 

 from the prism, without the use of 

 the screen. Thus the experiment (25) 

 in which the two spectra, lying in the 

 same line, were refracted by the prisms 

 to parallel lines, was repeated thus. The 

 spectra were viewed through a prism 

 without disturbing the screen, and their 

 apparent position, as seen through the 

 prism, was found to be the same as when 

 refracted by the third prism, and receive4 

 upon the screen. 



Two prisms ABC, A'B'C',/g-. 1 9, were 

 also placed at apertures in the window- 

 shutter, the refracting angle of one being 

 directed upwards, and that of the other 

 downwards. The spectra produced by 

 these prisms were both in an upright 

 position ; but had the colours arranged 

 in an opposite order, the red end being 

 the higher in the one, and the lower in 



