EVOLUTION IN THE SUNS 53 



above. The result is that what otherwise 

 would be a continuous bright spectrum is now 

 marked by dark lines corresponding to the 

 proper bright lines of the incandescent 

 condition of those elements that the light has 

 passed through. 



For example, in the case of the element 

 sodium, which occurs in common salt, it is 

 found that if a spectroscope be turned to a 

 colourless flame such as that of a spirit-lamp 

 or of a Bunsen burner, and some common salt 

 is then volatilized in the flame, so producing 

 a yellow flame, arising from the atomic 

 vibrations of the sodium atoms in the flame, 

 then, in the spectrum two bright yellow 

 lines close together are seen, and this is the 

 only spectrum visible. The reason is that 

 this is the only rate at which sodium atoms 

 produce optical vibration waves. If now the 

 spectroscope be turned towards the sun so as 

 to obtain the spectrum of sunlight, the 

 complementary phenomenon is seen. The 

 solar spectrum consists of a bright band of 

 colour ranging from red to violet, but it is 

 interrupted by a great series of thin black 

 lines, and the point of interest is that two of 

 these lines lying close together in the yellow 

 are in the exact position of the two sodium 

 lines of the sodium flame. In photographic 



