PRESIDENTIAL ADDRESS, B.A. 1871. 181 



Maxwell showed how, by aid of the spectroscope, 

 this idea may be made the foundation of a method 

 of measuring the relative velocity with which a 

 star approaches to or recedes from the earth. The 

 principle is, first to identify, if possible, one or 

 more of the lines in the spectrum of the star, with 

 a line or lines in the spectrum of sodium, or some 

 other terrestrial substance, and then (by observing 

 the star and the artificial light simultaneously by 

 the same spectroscope) to find the difference, if 

 any, between their refrangibilities. From this 

 difference of refrangibility the ratio of the periods 

 of the two lights is calculated, according to data 

 determined by Fraunhofer from comparisons 

 between the positions of the dark lines in the 

 prismatic spectrum and in his own " interference 

 spectrum " (produced by substituting for the prism 

 a fine grating). A first comparatively rough 

 application of the test by Miller and Huggins to a 

 large number of the principal stars of our skies, 

 including Aldcbaran, a Orionis, /3 Pcgasi, Sirius, 

 a Lyne, Capella, Arcturus, Pollux, Castor (which 

 they had observed rather for the chemical purpose 



