THE CONSTITUTION OF NEBULjE. 131 



(the sun and fixed stars), of scoriae or gradual formation of a cold and 

 non-luminous surface (the earth and planets), and finally of complete 

 gelation and torpidity (the moon), or whether they exist as a complete 

 and separate system of woi'lds; telescopes have only widened the 

 problem, and have neither simplified nor solved its difficulties. 



That which was beyond the power of the most gigantic telescopes 

 has been accomplished by that apparently insignificant, but really 

 delicate, and almost infinitely sensitive instrument the spectroscope ; 

 we are indebted to it for being able to say with certainty that lumi- 

 nous nebulae actually exist as isolated bodies in space, and that these 

 bodies are luminous masses of gas. 



The splendid edifice already planned by Kant in his " Theory of 

 the Heavens " (1755), and erected by Laplace forty-one years later, in 

 his " System of the Universe," has received its topmost stone through 

 the discoveries of the spectroscope. The spectroscope, in combina- 

 tion with the telescope, affords means for ascertaining even now some 

 of the phases through which the sun and planets have passed in their 

 process of development or transition from masses of luminous nebulae 

 to their present condition. 



Great variety is observed in the forms of the nebulae : while some 

 are chaotic and irregular, and sometimes highly fantastic, others ex- 



Fro. 3. 



The Large Magellanic Cloud. 



hibit the pure and beautiful forms of a curve, a crescent, a globe, or a 

 circle. A number of the most characteristic of these forms have been 

 photographed on glass at the suggestion of Mr. Huggins ; to these 

 have been added a few others, taken from accurate drawings by Lord 



