PHYSICAL SCIENCE 41 



Thus the chief difficulty in the wave theory of 

 light was overcome, while the phenomena of polarisa- 

 tion, seen when light is passed through such bodies 

 as crystals of Iceland spar, showed that the direction 

 of vibration of the waves must be at right angles 

 to the direction of propagation of the light. This 

 observation indicated some of the properties with 

 which the hypothetical aether of space must be en- 

 dowed if it be to serve as the medium through 

 which these waves can travel. 



The wave theory led directly to the explanation of 

 the black lines crossing the coloured band of light 

 or spectrum which is obtained by passing sunlight 

 through a glass prism. A theory of these lines had 

 been given by Sir George Stokes (1819-1903), but it 

 was not till the theory was reintroduced by the 

 German chemists Bunsen (1811-1898) and Kirchhoff 

 (1824-1887), and verified by experiment, that it was 

 generally accepted. A child's swing is set in motion 

 by giving it a series of gentle impulses in time with 

 its own natural period, and any mechanical system 

 will absorb energy which falls on it in similar periodic 

 unison with its own vibrations. The molecules of the 

 vapours in the outer envelope of the sun will absorb 

 the energy of particular rays coming from the hotter 

 interior when the waves coincide in oscillatory period 

 with the molecules. The light which passes on will 

 be deprived of those particular constituents that is, 

 of those particular coloured rays and black lines 



