on the Elementary Colours of the Spectrum. 493 



elsewhere speaking of Dr.Wollaston's observations, Dr. Young 

 observes, " Dr. Wollaston has determined the division of the 

 spectrum in a much more accurate maimer than had been done 

 before. * * * The spectrum formed in this manner consists of 

 four colours only, red, green, blue and violet. * * * The colours 

 differ scarcely at all in quality within their respective limits ; 

 but they vary in brightness, the greatest intensity of light being 

 in that part o^ \\\q, green which is nearest to the red"^-." 



Such is the composition of the daylight, or the blue sly, spcc- 

 t7-um, as we may call it. It has no ye/Zow space f. But in the 

 solar spectrum there is a distinct j/e/Zotu space of considerable 

 breadth lying between the red and green spaces, as clearly de- 

 fined in the beautiful di'awingof Fraunhofer. What then has 

 become of this j/^-Z/otu space in the daylight spectrum ? Some- 

 thing has been absorbed by reflexion from the sky or the clouds 

 which has reduced the j/eZ/ow light io green. This something is 

 a portion of red light ; because it is demonstrable, and it has 

 been maintained by Wollaston and Young, that red and gieeji 

 make yellow. Now this yellow space in the solar spectrum, 

 made yellowish-green in the daylight spectrum, may in its green 

 state be again madeyellcwX, and aho yellowish-white and white 

 by different absorbents. Hence it follows from Fraunhofer's, 

 and Wollaston's and Young's observations taken by themselves, 

 that the quality of the colours q/the brightest part of the spec- 

 trum is changed from yellow to yellowish-green, or green ac- 

 cording to Dr. Young. And it follows from my observations, 

 combined with theirs or when standing alone, that in the same 

 part of the spectrum there exist red, yellow, and blue rays of 

 the very same refrangibility. 



As M. Melloni does not seem to have read my reply to 

 the Astronomer Royal §, I shall merely refer him to it for a 

 notice of the experiments made by Sir John Herschel, which 

 confirm my analysis of the spectrum. That the quality of 

 the colour of the I'ed and ora?ige spaces is altered by absorp- 

 tion, is clearly proved by a casual experiment of Sir William 

 Herschel's ||. He found that " clear turned brass made the red 

 rays appear like orange, and made the orange colour different 

 from what it ought to be." 



Supported by the experiments and observations of such 



* Philosopliical Transactions, vol. i. p. 348. 



f I take no notice of the exceedingly narrow line of yellow light which 

 Dr. Young speaks of as generally seen at the limit of the red and green, and 

 to the mixture of which it is ascribed, because its existence or non-e.\ist- 

 ence does not affect my argument. 



+ Edinburgh Transactions, vol. ix. p. 442. 



§ Phil. Mag., March 1847, p. 133. 



II Phil. Trans., 1800, vol. xc. p. 255. 



