STELLAR SYSTEMS AND NEBULAE. 203 



(1832-1897), the famous American optician. 

 The period of the star seems to be about 50 

 years. In 1844 Bessel noticed irregularities in 

 the proper motion of Procyon, and put forward 

 the idea of a disturbing satellite, as in the case 

 of Sirius. This was confirmed by Madler, and 

 in 1874 an orbit was computed by Auwers, who 

 found a period of 40 years. In 1896 the 

 satellite was found by Schaeberle with the 

 36-inch refractor of the Lick Observatory. A 

 period of 40 years was found by See, in 

 agreement with the hypothetical orbit. 



In putting forward these theories as to in- 

 visible stellar satellites, Bessel remarked that 

 "light is no real property of mass," and that 

 the existence of countless visible stars is nothing 

 against the existence of countless invisible and 

 dark ones. In this he laid the foundation of 

 the branch of science termed by Madler the 

 " Astronomy of the invisible." In recent years 

 the astronomy of the invisible has become 

 a recognised branch of astronomical research, 

 through the application and interpretation of 

 Doppler's principle in spectroscopic observations. 

 In the course of photographing the stellar 

 spectra for the Draper Catalogue, E. C. Pick- 

 ering photographed the spectrum of Mizar 

 ( Ursae Majoris) in 1887 and again in 1889. On 



