CHAP. vi. NEW APPLICATIONS OF LIGHT. 45 



hydrogen lines only; and these are supposed to be hotter 

 than our sun, and in an earlier stage of development, 

 while red stars are supposed to be cooling. Other ex- 

 planations of these facts have, however, been suggested. 

 Much information has also been obtained as to the 

 nature of the nebulae. Sir William Herschel supposed 

 that they were all really star-clusters, but so enormously 

 remote that even the most powerful telescopes could not 

 render visible the stars composing them. Later observa- 

 tions have shown that many of them do consist of stars, 

 or star-dust, as it has been called ; and this seemed to sup- 

 port the theory that all were so composed, including the 

 milky way. A study of the distribution of stars and 

 nebulae by Proctor and others led, however, to the con- 

 clusion that they were often really connected, and that 

 nebulae were not, on the average, more distant than stars ; 

 and this vie\v has been confirmed by the spectroscope, 

 which has shown them often to consist of glowing gas; 

 and this is especially the characteristic of all those situ- 

 ated in or near the milky way. The first great result of 

 spectrum-analysis has thus been to demonstrate the real 

 nature of many stars and nebulae, to determine some of 

 the elements of which they are formed, and to give us 

 some indications of the changes they have undergone, 

 and thus help us toward a general theory of the develop- 

 ment of the stellar universe. 



Marvellous as is this extension of our knowledge of 

 objects so distant that our largest telescopes are power- 

 less to show them as more than points of light, it is only 

 a part, perhaps only a small part, of what the spectro- 

 scope has already done, or may yet do, for astronomy. 

 Bv a most refined series of observations it has enabled us 



