XXI.] 



THE SUN. 



369 



which the light is admitted, and C is the cross-section of the 

 prism : instead of faUing as white light at B, the beam is 

 deflected from its original path, and widens into a many- 

 tinted band, DE, with red at one end and violet at the other 

 extremity. 



On closely examining the spectrum of the sun, which 

 has thus been formed through a narrow slit, it is found to be 

 crossed by a multitude of fine dark lines, which are indeed 

 so many spaces in the bright band. A spectrum obtained 

 from an ordinary gas-flame, or from the electric light, differs 

 from the solar spectrum by being destitute of these dark 



Fig. 121. — Formation of the solar spectrum. 



lines ; the light of the flame being unbroken from end to 

 end. But, if certain gases or vapours, such as hydrogen or 

 sodium-vapour, be burnt in the path of the artificial light, 

 lines are immediately produced in the spectrum. If the 

 temperature of the substance which produces the lines be 

 lower than that of the substance which gives the continuous 

 spectrum, the lines will appear dark ; if the temperature be 

 higher, the lines appear bright. Lines produced in this 

 way have a definite position in the spectrum, so that the 

 same chemical element, under the same circumstances, always 

 gives the same set of lines. It is plain, therefore, that, by 

 observing the position of the lines in the solar spectrum, 



B B 



