PHYSICS OF NINETEENTH CENTURY 175 



selves give out when heated. Thus, when one of 

 Fraunhofer's lines coincides with the bright line in the 

 laboratory spectrum .of a terrestrial substance, the 

 presence of that element in the envelope of the sun is 

 to be inferred. Moreover, by passing the intensely 

 white light of the electric arc through sodium vapour, 

 volatilized in the cooler flame of a spirit-lamp, it 

 should be possible to reproduce artificially the dark 

 absorption line of sodium. Such an experiment indeed 

 had been carried out by Le*on Foucault in 1849, but 

 had passed unnoticed and been forgotten. It was 

 performed once more by Bunsen and Kirchhoff in 

 1860, in the light of the theory given above, and its 

 success at once opened up the new science of the 

 chemistry of the sun and stars. By analysis of their 

 light, it became possible to detect the existence in 

 the depths of space of elements known on our globe. 

 Moreover, since the colour of light and the relative 

 position of spectral lines depends on the frequency 

 of the vibrations which reach the eye, a luminous 

 source approaching an observer will crowd more 

 waves into his eye per second than if the source were 

 at rest. Thus a slight shift, in one direction or the 

 other, in the position of spectral lines which can be 

 measured microscopically indicates relative approach 

 or retreat between the source of the light and the earth. 

 In this way the velocities and movements of stars 

 can be investigated by a study of the light which they 

 emit. Although many stellar spectra are too faint 

 to affect the eye, in photography small effects can 

 be made cumulative, so that, by long exposure, lines 

 invisible to the unaided eye can be detected on a 

 photographic plate. 



