MIRACLES OF SCIENCE 



that the spectroscopic lines were doubled period- 

 ically, every 52 days, and the only justifiable infer- 

 ence from this is that the star is a binary. Mizar was 

 previously known as a telescopic double, but the 

 second component of the spectroscopic binary is not 

 the one that can be seen with the telescope. In 

 other words, one of the visible components of the 

 spectroscopic binary is itself a double star, the parts 

 of which are too close together to be separated even 

 with the most powerful instrument. 



There is yet another type of spectroscopic binary, 

 in which one component is a dark star, and in which 

 the orbital shift of the bright companion is revealed 

 by a backward and forward movement of the spec- 

 troscopic lines. To detect this shift it is necessary 

 to use what is called a slit spectroscope at the eye- 

 piece of the telescope and a comparison spectrum. 

 The first binary of this type was discovered by Vogel 

 at Potsdam, also in 1889, the star first observed be- 

 ing the famous variable Algol. Vogel found that 

 the lines of this star shift back and forth in a period 

 precisely conforming to the period of its variability 

 (two days, 20 hours, 49 minutes). 



It had long been supposed that the observed varia- 

 bility of Algol must be due to the periodical partial 

 eclipse of the bright star by a dark companion which 

 chanced to be revolving in an orbit precisely in our 

 line of sight. But this remained a pure conjecture 

 until the spectroscopic test proved that the star is 

 a binary revolving in an orbit with a dark com- 

 panion. Moreover, the shift of the spectroscopic 

 lines agreed perfectly in point of time with the re- 



