ASTRONOMY AND COSMIC THEORIES. 105 



radial or spiral mode of condensation, and to others 

 which possess a dense nucleus like a comet; then we 

 have the compact discs called planetary nebulae, and 

 others which seem to be aggregated around one or more 

 bright stars. Recently it has been found that many 

 stars, among others those of the Pleiades, which ap- 

 pear as stars only in the most powerful telescopes, are 

 really nebulous stars when photographed with very long 

 exposure under conditions which exhibit many thou- 

 sands of stars which no telescope can render visible. 

 And when these various bodies are examined with the 

 spectroscope, they are seen to have many features in 

 common, such as indicate differences in temperature and 

 consequent difference in the amount and character of the 

 luminous gases due to their greater or less condensation. 

 The nebulae of various forms and intensity represent, 

 therefore, the early stages in the development of stars, 

 suns, and planetary systems out of diffused meteoritic 

 matter; while stars themselves are of various tempera- 

 tures, the heat increasing when the meteoritic matter is 

 most rapidly aggregating, and afterward cooling till 

 they become of so low a temperature as to cease to be 

 luminous to our vision, as is the case with the dark com- 

 panions of some of the spectroscopic double stars. 



This conception of the meteoritic constitution of the 

 whole stellar universe is one of the grandest achieve- 

 ments of the science of the nineteenth century. All 

 the other astronomical discoveries of the period (except 

 those gained through the spectroscope) are additions to 

 our knowledge of essentially the same nature as others 

 which preceded them ; but in this case we have a new and 

 comprehensive generalization, which links together a 



