322 DISCOVERY CH. 



brightness, if Goodricke's explanation of the cause of 

 variability is correct. 



The spectroscope has proved that these periodic back 

 and forth movements of Algol do occur. By means of 

 this instrument, the velocity of approach or recession 

 of a star, with reference to the earth, can be determined 

 within a fraction of a mile a second ; and in the year 

 1888 Prof. H. C. Vogel, of the Potsdam Observatory, 

 showed that Algol recede , from the earth with a velocity 

 of about twenty-four miles a second before its light 

 dims, and approaches us at the rate of about twenty- 

 eight miles a second, after having been thus partially 

 eclipsed. Taking the average velocity as 26| miles a 

 second, the number of miles traversed around the 

 balancing point in the whole period can be easily 

 calculated, and thus the diameter of the orbit can be 

 obtained. Knowing these dimensions, and the period 

 of revolution, Kepler's third law (see p. 142) enables 

 the mass of Algol and its companion to be determined 

 in comparison with that of the sun. The conclusion 

 arrived at by this reasoning is that the dark companion 

 of Algol has about the same diameter as the sun, while 

 the star itself is somewhat larger. 



The dark companion has never been seen, and probably 

 never will be, yet there is not the slightest doubt of its 

 existence among astronomers, nor can there be in any 

 mind that has considered the testimony given by the 

 spectroscope. About twenty stars are known to fluc- 

 tuate in light in the same manner as Algol, and each of 

 them is regarded as having a dark satellite which 

 periodically comes between us and the luminous 

 star. 



The condition of things represented by Algol and 

 other stars of the same type must, however, be excep- 



