CH. xxxiil. THE SOLAR SPECTRUM. 325 



the two spectra, 2 and 3, cover each other), the yellow line 

 exactly filled up the black line with its light. He now 

 wished to see how bright he could make the solar spectrum 

 without overpowering the light of the sodium, so he let the 

 full sunshine pass through the sodium flame. To his great 

 astonishment he saw the black line at D start out more 

 strongly than ever. The sodium flame had revenged itself 

 for being overpowered by absorbing some of the yellow light of 

 the sun ! 



This suggested to him the idea that the black line D must 

 be caused by the white light from the sun passing through 

 sodium vapour before it reaches us. There was a very simple 

 way of proving whether this were so ; for burning solids, you 

 remember, give a continuous spectrum (i, Plate I.), there- 

 fore, if he could produce a dark line by passing the light of 

 a burning solid through sodium vapour, he would imitate 

 one of the defects in sunlight So he burned a lime-light, 

 and when he had the continuous coloured band in his 

 spectroscope, he burned a sodium flame between the lime- 

 light and the prism. The experiment was quite successful ; 

 the dark space, D, started out upon the spectrum, and thus 

 he proved beyond doubt that burning sodium vapour absorbs 

 in white light exactly those rays which it gives out itself when 

 burning. 



He repeated the experiment with other burning metals, 

 such as potassium and strontium, and always with the 

 same result. Each burning gas absorbed in the white light 

 exactly those rays which it gave ottt itself when burning. 



The black lines on the solar spectrum were now explained, 

 for each one of them must imply that some particular ray of 

 sunlight has been absorbed by a gas between the sun and 

 us, and it must hqve been absorbed near the sun, as Fraun- 



