174 SCIENCE AND THE HUMAN MIND 



of propagation. Finally, since the hypothesis of 

 waves certainly connotes the idea of something to 

 vibrate, it was thought necessary to revive the old 

 conception of an all-pervacl'ng universal aether to 

 supply what the late Lord Salisbury once called " a 

 nominative case to the verb ' to undulate.' ' 



The wave theory gave an explanation of the 

 phenomena which led to the triumphs of spectrum 

 Spectrum analysis. Joseph Fraunhofer (1787-1826) 

 Analysis, had mapped the black lines which cross 

 the coloured band of light, or spectrum, which is 

 obtained by passing a beam of parallel rays of sunlight 

 through a glass prism. The nature of these lines seems 

 first to have been made plain by Sir George Gabriel 

 Stokes (1819-1903) in his lectures at Cambridge, though 

 with characteristic modesty he gave his ideas no wider 

 publicity. A child's swing is set in motion by giving 

 it a series of small impulses which coincide with 

 its natural period of oscillation, and, in similar manner, 

 any mechanical system will absorb energy which falls 

 on it in rhythmic unison with its own natural vibra- 

 tions. Hence the molecules of the vapours in the outer 

 envelope of the sun will absorb the energy of those 

 particular rays coming from the hotter interior, of 

 which the oscillatory period coincides with their own. 

 The light which passes on, then, will be deprived 

 of light of that particular colour (i.e. frequency of 

 vibration), and a black line in the solar spectrum is 

 the result. Bunsen and Kirchhoff saw that, if this 

 explanation be the true one, the black lines due to 

 absorption should correspond in position with the 

 bright lines the molecules of the same element them- 



