Treqear. — Inaugural Address. 607 



it is thought certain that many of the fixed stars have dark 

 planets revolving round them. This fact has been affirmed by 

 observations on a class of stars known as Algol-variables, 

 and they are so named after the star Algol, in the con- 

 stellation Perseus. This star is ordinarily of the second mag- 

 nitude, but at regular intervals of time, in a little less than 

 three days, it sinks for an hour or two to the fourth mag- 

 nitude, then resumes its former brightness, the whole change 

 occupying about five or six hours. The fact that there are 

 numerous stars of this character is now fully established ; their 

 brightness remains constant for a time, then suffers partial 

 eclipse for a few hours, and always at regular intervals. 

 It was long ago suspected in the case of Algol that this 

 eclipse was caused by the revolution of a dark body round 

 the star, and was fairly well proved by Vogel's measure- 

 ment of the motion of that star in the line of sight ; it is a 

 natural conclusion that the similar and regular eclipse of other 

 stars is due to a like cause. A planet could not be seen to 

 eclipse a star unless our system lies near the plane of its 

 orbit, therefore it is only reasonable to conclude that there are 

 many other planets not moving in such a plane. The eclipses 

 of these planets would not be visible to us, as in such case 

 they would pass either above or below the star. To give some 

 idea of the care and incessant watchfulness necessary for as- 

 tronomical record it should be pointed out that observations 

 on these Algol-variables have to be made during the time of 

 partial eclipse, and compared with observations at other times. 

 An observer might record the magnitude of such a star on a 

 dozen occasions, and yet never happen to strike the period of 

 obscuration. This new addition to our knowledge proves that 

 there can be no limit to what we yet may learn concerning the 

 suns and worlds in space. Thirty years ago no project seemed 

 more hopeless than that of detecting an invisible planet re- 

 volving round some immensely distant sun, but now it is a 

 well-cultivated branch of astronomy. 



Another late discovery is that of the companion of the 

 brilliant star Procyon. This star, like Sirius, was thought to 

 have a companion, because by very refined observations of 

 position the visible star was found to revolve round a fixed 

 centre. The companion of Sirius was discovered by Alvan 

 Clark, the companion of Procyon by Schaeberle. Procyon's 

 satellite is found to have a mass of about one-half of its bright 

 star, and the revolution takes about forty years. The com- 

 panion of Sirius has a mass of about one-third of its brilliant 

 ally, and its revolution is in fifty years. The systems of 

 planets moving about such stars seem to have more eccentric 

 orbits than the planets of our own solar system. 



It is one of the triumphs of spectrum analysis to have dis- 



