THE BINARY STARS. 325 



Algol has again attained his maximum brilliancy, and this he 

 retains for another period of 2 days 13^ hours, after which he 

 goes through exactly the same changes again. The theory of a 

 partial eclipse by a dark companion was a perfectly feasible one, 

 but it could not be proved without the spectroscope, as no telescope 

 would show Algol as a disc of any appreciable size, but merely 

 (like all the other fixed stars) as a point of light. This could be 

 seen to vary in brightness, but of course not in shape, as would 

 be the case if we were near enough to see the passage of a dark 

 satellite in front, partially eclipsing the star. This we can never 

 hope to actually see, but the spectroscope enables us to see it with 

 our mind's eye, and with little less certainty of its truth than if 

 we saw it in reality. 



Professor Vogel first applied the spectroscope to Algol, and he 

 then found that the star was approaching the earth at a speed of 

 something like twenty-six miles a second. When next he tried it 

 he found that Algol was receding from us at about twenty-six miles 

 a second. He tried again and again, and found that the star was 

 •really moving with a maximum velocity of twenty-six miles a 

 second, sometimes towards the earth and sometimes away from 

 us, and sometimes it appeared to stand still. Vogel found that, 

 at the conclusion of the period of approach, Algol appeared to 

 be stationary. Then a slow movement of retreat began, which 

 gradually increased till a speed of twenty-six miles a second was 

 attained. Then this gradually declined until he appeared again 

 to stand still. Then a slow movement of approach began, which 

 also gradually attained a maximum speed of twenty-six miles a 

 second, and then slowly decreased until again no movement at 

 all was perceptible. 



Now, it is quite impossible to suppose that Algol can run 

 along in a straight line with first accelerating and then diminishing 

 velocity, and stop and turn back at each end of his course. Such 

 a movement would be quite at variance to all astronomical move- 

 ments with which we are acquainted, and it is impossible to invent 

 any system of forces which would produce or explain such an 

 erratic movement on the part of one of the stars ; yet this is what 

 the spectroscope at first sight declared Algol to be doing But it 

 must be borne in mind that the spectroscope cannot show move- 



