ADVANCE OF ASTRONOMY. 197 



tation, it appears certain that in coming ages their 

 glowing matter must, under its influence, be drawn 

 towards centres of condensation; the smaller and 

 more symmetrical of the nehuloe possibly developing 

 into single stars, but such majestic collections of 

 cloudy structures as are revealed in Orion being more 

 probably the origin of hosts of separate suns." 



Dead Stars. — While some nebulae are plausibly 

 interpreted as stars a-making, there are also phe- 

 nomena which indicate stars dying or dead, or in 

 other words, dark. It is obvious that the existence 

 of a dark star cannot be demonstrated to the eye ; but 

 it may be inferred (a) from the occurrence of the 

 total or partial eclipse of a bright star, or (6) from 

 disturbances in the movement of a bright star such 

 as the gravitational influence of a dark neighbour 

 would explain. In both these ways the existence of 

 dark stars has been indirectly proved. 



The regularity in the variations of the light of 

 Algol — the best-known of the variable stars — was 

 hypothetically interpreted by Goodriche (1782) as 

 due to the revolutions of a dark companion star 

 which caused partial eclipse; and the researches of 

 E. C. Pickering of Harvard (1888) and of Vogel of 

 Potsdam (1888-1891) have justified the hypothesis. 



" The possibility of an unseen system of stars per- 

 meating the seen is beyond doubt.^' * 



Condensing Dr. Fison's account of the subject, we 

 may sum up the possible history of a nebula. A 

 diffused area of gases, perhaps comparatively cool, 

 perhaps holding part of its contents in the form of 

 solid or liquid particles; gravitational attraction 

 brings about a spherical form; heat is lost by radi- 

 ation and the parts of the area draw together; tem- 

 *Fison. Recent Advances, p. 35. 



