186 A CENTURY'S PROGRESS IN ASTRONOMY. 



of the star's light corresponds with the obscura- 

 tion of the lesser star, the lesser minimum with 

 that of the primary, implying that the primary 

 is much less luminous in proportion to its light 

 than its satellite, a state of affairs which Miss 

 Clerke concludes to be improbable. 



The variable stars, 8 Cephei and T? Aquilae, 

 were both found in 1894 by Belopolsky to be 

 binaries ; but as the times of minimum light 

 do not correspond with those of eclipses in 

 the hypothetical orbits, he concludes that the 

 variations cannot be explained on the eclipsing 

 satellite theory. Miss Clerke is inclined to the 

 theory that the increase of luminosity in short- 

 period variables is due to tidal action, so that 

 while the revolutions of the stars control their 

 variability, they are inherently unstable in light. 

 A large number of these stars are known, and 

 it is a remarkable fact that the majority of these 

 variables lie on or near the Galaxy, so that their 

 variations have probably something to do with 

 their vicinity. 



We now come to the long-period variables of 

 which Mira Ceti, x Cygni, and U Orionis are 

 examples. Although varying in regular periods, 

 generally of about a year, they are subject to 

 remarkable irregularities, so that an exact period 

 cannot be assigned even to Mira Ceti, of which 



