PHYSICAL SCIENCE. 



xxxvii 



Wished between the structure of crystallized 

 bodies, and the property of double refraction , the 

 theory must remain imperfect. 



We now come to the discoveries of Newton, 

 which constitute nearly as important an era in 

 optical science as his theory of universal gravita- 

 tion did in astronomy. His researches began in 

 1666, when he was only twenty-three years of 

 age. 



In hopes of improving the telescope, by giving 

 to the glasses a figure different from the spheri- 

 cal, he had procured a glass prism, in order, as 

 he tells us, to try with it the celebrated pheno- 

 mena of colours. These trials led to the discovery 

 of the different refrangibility of the rays of light, 

 now too well known to require to be par- 

 ticularized. 



Having admitted a ray of light into a dark 

 chamber through a hole in the window-shutter, 

 and made it fall on a glass prism, so placed as to 

 cast it on the opposite wall, he was delighted to 

 observe the brilliant colouring of the sun's 

 image, and not less surprised to observe its 

 figure, which, instead of being circular, was 

 oblong in the direction perpendicular to the 

 edges of the prism, so as to have the shape of a 

 parallelogram, rounded at the two ends, and 

 n early five times as long as it was broad. 



When he reflected on these appearances, he 

 saw nothing that could explain the elongation of 

 the image, but the supposition, that some of the 

 rays of light, in passing through the prism, were 

 more refracted than others ; so that the rays 

 which were parallel before they fell on the prism, 

 being some of them more refracted than others, 

 diverged from one another after refraction, the 

 rays that differed in refrangibility differing also 

 in colour. The spectrum would thus consist of 

 a series of circular images, partly covering one 

 another, and partly projecting one beyond the 

 other, from the red, or least refrangible ray, in 

 succession to the orange, yellow, blue, indigo, 

 and violet, the most refrangible of all. 



But he did not adopt this as the true explana- 

 tion, till he had tried every other hypothesis by 

 the test of experiment, and proved its fallacy. 

 Even after these rejections, his explanation had 

 still to abide the sentence of an experimentum 

 crucis. 



Having admitted the light, and applied the 

 prism as before, he received the coloured spec- 

 trum on a board about twelve feet distant from 

 the prism, and pierced with a small hole. The 

 coloured light, which passed through this second 

 hole, was made to fall on a prism, and after, 

 wards received on the opposite wall. It Avas 

 then found, that the rays which had been most 

 refracted by the first prism were also most re- 

 fracted by the second, though no new colours 



were produced. It was also observed, that when 

 the rays which fell on the second prism were all 

 of the same colour, the image formed by refrac- 

 tion was truly circular, and of the same colour 

 with the incident light. 



When the sun's light is thus admitted, first 

 through one aperture, and then through another 

 at some distance from the first, and is afterwards 

 made to fall on a prism, as the rays come only 

 from a part of the sun's disc, the spectrum has 

 nearly the same length as before ; but the breadth 

 is greatly diminished ; in consequence of which, 

 the light at each point is purer, it is free from 

 penumbra, and the confines of the different 

 colours can be more accurately traced. It was 

 in this way that Newton measured the extent of 

 each colour; and taking the mean of a great 

 number of measurements, he assigned the follow- 

 ing proportions, dividing the whole length of the 

 spectrum, exclusive of the rounded terminations, 

 into 360 parts. Of these the 



Red occupied ... 45 



Orange .... 27 



Yellow .... 48 



Green . . . . 60 



Blue 60 



Indigo . . . .40 



Violet 80 



Norton conceived that there Avas an identity 

 between these numbers and the divisions of the 

 monochord, by the notes of music. But it has 

 been since observed, that the spaces occupied by 

 these colours differ according to the kind of 

 prism employed. Hence the relation was merely 

 accidental 



Thus colours were found to be original pro- 

 perties of light, connected with the different de- 

 grees of refrangibility belonging to the different 

 rays. Each of these colours, though primary, 

 may be produced by a mixture of the two colours 

 upon either side of it in the prism. Red and 

 yellow make orange; yellow and blue make 

 green ; and so on. In general, two colours not 

 very far distant in the natural series, when 

 mixed, make up the intermediate colour. All 

 of them together constitute whiteness. Natural 

 bodies of whatever colour, if viewed by simple 

 and homogeneous light, are seen of the colour of 

 that light, and no other. 



Thus supplied with so many new and accurate 

 notions respecting colours, it was natural for 

 Newton to apply them to explain the rainbow. 

 Every thing respecting that beautiful pheno- 

 menon had been already explained, except the 

 colours. As these colours were the same as in 

 the solar spectrum exhibited by the prism, and 

 in the same order, it could hardly be doubted 

 that they were owing to the same cause. He 

 showed this to be the case, by calculating the ex- 



