42 THE FOUNDATIONS OF SCIENCE 



corresponding to this omission will cross the solar 

 spectrum. 



These conditions were reproduced in the laboratory 

 by Bunsen and Kirchhoff, who passed the intense 

 white light of an electric arc through sodium 

 vapour volatilised in the cooler flame of a spirit 

 lamp, and obtained the black absorption line of 

 sodium in the resulting spectrum. 



By this discovery the whole new field of the consti- 

 tution of the sun and stars was opened to chemistry, 

 which found proof of the existence of its familiar 

 terrestrial elements in the depths of space. More- 

 over, spectrum analysis soon detected new elements 

 both on the earth, where rare alkalies came to light, 

 and in the sun, where an unknown spectral line 

 suggested a new element, helium, only afterwards 

 detected on the earth in the gases hidden in certain 

 minerals. 



But these methods of research have done more than 

 reveal the chemical nature of celestial bodies. If the 

 source of light and the observer are approaching each 

 other, more waves per second reach the eye than if 

 they were relatively at rest. Hence the colour of a 

 simple ray and its position in the spectrum are slightly 

 changed. By measuring microscopically such dis- 

 placement of known lines in the spectra of stars, their 

 relative velocity of approach or recession in the line 

 of sight may be estimated a feat which might well 

 have been thought impossible of attainment. 



