CH. xxxin. SPECTRA OF DIFFERENT FLAMES. 321 



sents one of the black lines in the spectrum where a dark 

 image of the slit is thrown, and if you take out those which 

 correspond to the lines in the sun spectrum No. 2, you will 

 have an illustration of ' Fraunhofer's lines.' 



Fraunhofer measured these black lines with the greatest 

 care, and he found that in every ray of sunlight they came 

 exactly in the same places. Then he tried the light of. 

 the moon and Venus ; still the black lines were the same, 

 for these planets, as you know, only shine by the light of 

 the sun. But, when he turned his telescope to the stars 

 and caught their light, he found a difference. There were 

 dark lines in the star-spectrum, but they were not all in 

 the same place as those in the sun-spectrum, as you will 

 see if you compare No. 2, Plate I., with the star-spectrum, 

 No. 5, in which the lines seen on the right-hand side of the 

 solar spectrum are entirely wanting. 



Fraunhofer, therefore, argued in this way : If the black 

 spaces were caused by some of the waves being stopped in 

 coming through our own atmosphere, they would be the 

 same in any spectrum wherever the light came from. But 

 as these dark spaces are different in the starlight from what 

 they are in the sunlight, there must be some real difference 

 between the light of the sun and the light of the stars before it 

 comes to us. This was the first step in the study of the 

 heavenly bodies by means of spectrum analysis. 



Experiments on the Spectra of different Flames, 1822. 

 For more than forty-five years these black lines remained a 

 complete puzzle to all who studied the spectrum, but in the 

 meantime Sir John Herschel, Mr. Fox Talbot, Sir David 

 Brewster, and others, had made many valuable experiments 

 upon the colours produced by different burning lights. You 

 know already that it is possible to make coloured flames by 



