792 THE POPULAR SCIENCE MONTHLY. 



represented on the map are worth examining, although none of 

 them calls for special mention, except perhaps 584, where we may 

 distinguish at least a hundred separate stars within an area less 

 than one quarter as expansive as the face of the moon. 



Among the double stars of Perseus we note first 77, whose com- 

 ponents are of magnitudes four and eight, distance 28", colors 

 white and pale blue. The double e is especially interesting on 

 account of an alleged change of color from blue to red which the 

 smaller star undergoes coincidently with a variation of bright- 

 ness. The magnitudes are three and eight, distance 9", p. 9. An 

 interesting multiple is , two of whose stars at least we can see. 

 The magnitudes are three, nine, ten, and ten, distances 13", p. 

 207, 90", and 112". 



The chief attraction in Perseus is the changeful and wonderful 

 /?, or Algol, the great typical star among the short-period vari- 

 ables. During the greater part of its period this star is of magni- 

 tude two and two tenths, but for a very short time, following a 

 rapid loss of light, it remains at magnitude three and seven tenths. 

 The difference, one magnitude and a half, corresponds to an ac- 

 tual difference in brightness in the ratio of 3"75 to 1. The entire 

 loss of light during the declension occupies only four hours and a 

 half. The star remains at its faintest for a few minutes only 

 before a perceptible gain of light occurs, and the return to maxi- 

 mum is as rapid as was the preceding decline. The period from 

 one minimum to the next is two days twenty hours forty-eight 

 minutes fifty-three seconds, with an irregularity amounting to a 

 few seconds in a year. The Arabs named the star Algol, or the 

 Demon, on account of its eccentricity which did not escape their 

 attention ; and when Goodricke, in 1782, applied a scientific meth- 

 od of observation to it, the real cause of its variations was sug- 

 gested by him, but his explanation failed of general acceptance 

 until its truth was established by Prof. E. C. Pickering in 1880. 

 This explanation gives us a wonderful insight into stellar consti- 

 tution. According to it, Algol possesses a companion as large as 

 the sun, but invisible, both because of its proximity to that star 

 and because it yields no light, and revolving in a plane horizontal 

 to our line of sight. The period of revolution is identical with the 

 period of Algol's cycle of variation, and the diminution of light 

 is caused by the interposition of the dark body as it sweeps along 

 that part of its orbit lying between our point of view and the 

 disk of Algol. In other words, once in every two days twenty 

 hours and forty-nine minutes Algol, as seen from the earth, un- 

 dergoes a partial eclipse. 



In consequence of the great comparative mass of its dark com- 

 panion, Algol itself moves in an orbit around their common cen- 

 ter with a velocity quite sufficient to be detected by the shifting 



