Spectrum Analysis 155 



employed. These differences are so obvious that any one 

 metal may be instantly distinguished from the others by the 

 appearance of its spark, and we have here a mode of dis- 

 criminating metallic bodies more ready even than chemical 

 examination, and which may be hereafter employed for 

 useful purposes." Wheatstone himself fully realized the im- 

 portance of the subject, as is shown by his remark that " the 

 peculiar effects produced by electrical action on different 

 metals depend, no doubt, on molecular structure, and con- 

 tain hence a new optical means of examining the internal 

 mechanism of matter." 



So much for what was known of the emission spectra of 

 luminous bodies before the date of Kirchhoff and Bunsen's 

 work; let us now turn to the phenomena of absorption. 

 Wollaston was the first who mentioned the dark lines which 

 traverse the spectrum of solar light, but he seems to have 

 looked upon them mainly as lines separating the different 

 colours, though he points out two of them that were not. 

 During the researches which Fraunhofer, the famous 

 optical instrument maker of Munich, conducted with a view 

 to improving the methods of determining the refractive 

 indices of different kinds of glass, sunlight was examined, 

 and found to contain many fine dark lines in its spectrum ; 

 these are now called " Fraunhofer lines." A large number 

 of them were carefully mapped, and the most prominent 

 served him as standards for his measurements; but he 

 examined also the light of a luminous flame and that of 

 some of the stars and planets. The first experiments date 

 back to 1814; nine years later he returned to the subject 

 and measured the wave-lengths of the principal lines by 

 means of his gratings. He pointed out that by using a blow- 

 pipe he could obtain a flame which emits a close doublet 

 of yellow light coincident with the solar lines D. Fraunhofer 

 examined the spectrum of the " electric light," and noticed 

 bright lines; he used the spark of an electric machine as 

 source of illumination and apparently took what we now 

 know to be the spectrum of air as characteristic of the electric 

 source of illumination. Of greater importance are his 

 observations on the spectra of the stars and planets, which 

 allowed him to recognize that the planets, like the moon, 



