STUDY OF THE VARIABLE STAES 185 



its brightness. The whole time for the decrease and the restoration of 

 light is about ten hours. The form of this light-curve points unmis- 

 takably to the eclipse of a bright star by a relatively dark companion. 

 This explanation, first proposed by Goodricke, was developed by Picker- 

 ing, and proved spectroscopically by Vogel. Dr. Alexander W. Roberts, 

 of Lovedale, South Africa, has recently developed a method for de- 

 termining the absolute dimensions of an Algol binary. The theory 

 which underlies the determination is that light takes an appreciable 

 interval of time to traverse the orbit of a binary system. For an 

 accurate solution observations of the highest precision are essential. 

 Precise photometric observations of such objects have been made by 

 Professor 0. C. Wendell, of the Harvard Observatory. The cause of 

 variation is in general the same for all the Algol variables, though 

 there are minor differences of importance. As might be expected, they 

 show great regularity. Nevertheless, there are certain secular varia- 

 tions from causes not well understood. The period of Algol is believed 

 to vary slightly, and Dr. Chandler explains this as due to the presence 

 of a third body. M. Tisserand, however, has advanced a different 

 theory. He assumes a slight flattening of the globe of Algol, and an 

 elliptical orbit for the companion. These rival theories can be settled 

 only by elaborate determinations of the light-curve during many years. 

 According to Dr. Chase, Algol is at a distance of ninety-three light- 

 years. Vogel finds the diameter of each of the components to be nearly 

 a million miles, and the distance between them little more than three 

 million miles. There are doubtless thousands of binary systems in the 

 heavens, one component of which is more or less obscure. Such a sys- 

 tem, and it holds true even if the components are equally luminous, 

 becomes for us an Algol variable when the plane of revolution passes 

 through or near the earth. Such systems are comparatively rare. At 

 the present time only thirty-eight are known. The largest variation 

 yet found is that of Fleming's Algol, E. W. Tauri, whose light at mini- 

 mum is only one twenty-sixth as great as its usual amount. It would 

 be possible for a dark companion of the same size as the bright com- 

 ponent to completely eclipse it. In the case of v Cephei, indeed, this 

 probably takes place, so that the light while the eclipse lasts comes 

 entirely from the dark companion. The companion is only relatively 

 dark, however, so that its light alone is about one eighth as great as 

 the combined light of both components. If the companion, in such a 

 case, were completely obscure, there would be a total eclipse of the 

 star's light, but no such case has yet been found. 



