322 L. Page — A Century's Progress in Physics. 



sun in the infra-red in amounts far exceeding that con- 

 tained in the visible spectrum. In 1842 Doppler drew 

 attention to the fact that motion of the source should 

 cause a displacement of the spectral lines, the shift being 

 to the blue if the light is approaching and to the red if 

 it is receding, and a few years later Fizeau suggested the 

 application of Doppler 's principle to the measurement of 

 the velocity of a star moving in the line of sight. Thus 

 the spectroscope has been able to supply one of the 

 deficiencies of the telescope, and the two together are 

 sufficient to reveal all components of stellar motion. 

 When spectra formed by light from the sun's limb and 

 from its center are compared, the same effect reveals 

 the rotation of the sun about its axis. (C. S. Hastings, 5, 

 369, 1873; C. A. Young, 12, 321, 1876.) 



Further Evidence of the Electron. — In 1845 Faraday 

 discovered a rotation of the plane of polarization when 

 light passes in the direction of the lines of force through 

 a piece of glass placed between the poles of an electro- 

 magnet. Examination of the spectrum from a glowing 

 vapor situated between the poles of a magnet, however, 

 failed to reveal any effect of the field. The latter prob- 

 lem was attacked anew by Zeeman 7 in 1896, and with the 

 aid of the improved appliances of modern science he suc- 

 ceeded in detecting a broadening of the lines. Later 

 experiments with more powerful apparatus resolved 

 these broadening lines into several components. 



Lorentz 8 showed at once how the electron theory fur- 

 nishes an explanation of the Zeeman effect. He found 

 that when the source is viewed at right angles to the lines 

 of magnetic force, a spectral line should be split into 

 three components. Of these he predicted that the mid- 

 dle, or undisplaced component, would be found to be 

 polarized at right angles to the direction of the field, and 

 the other components parallel to the field. When the 

 light proceeds from the source in a direction parallel to 

 the magnetic lines of force, two components only should 

 be formed, and these should be circularly polarized in 

 opposite senses. Moreover, from the separation of the 

 components can be calculated the ratio of charge to mass 

 of the electronic vibrator which is responsible for the 

 emission of radiant energy. Zeeman 's experiments cou- 



7 P. Zeeman, Phil. Mag., 43, 226, 1897. 



8 H. A. Lorentz, Phil. Mag., 43, 232, 1897. 



