THOUGHTS ON NATURAL PHILOSOPHY. 33 



present. The light from the flame of a spirit-lamp 

 which has acquired a strong yellow tint by sprinkling 

 a trace of common salt upon the wick, is, for instance, 

 resolved into two closely coincident shades of yellow, 

 indicated in the spectroscope by the appearance of a 

 pair of closely adjacent yellow lines; and the peach- 

 coloured glow emitted by hydrogen gas when rendered 

 luminous by a discharge of electricity through it, 

 gives rise to the appearance of several coloured lines, 

 of which a crimson and an emerald-green appeal 

 most strongly to the eye. 



In the year 1864, Sir William Huggins first applied 

 the spectroscope to the study of the nebulae, the par- 

 ticular one selected being a small but comparatively 

 bright object in the constellation of the Dragon. 

 The light from the nebula was condensed upon the 

 slit of the spectroscope by the object-glass, eight 

 inches in diameter, of an astronomical telescope; 

 and at the first glance, the examination of the 

 spectrum showed it to be characteristic of the light 

 emitted from a glowing gas, since it consisted, not 

 of a continuous band, but of three separated lines, 

 all of them being of a green colour. The luminous 

 matter of the nebula consisted, therefore, not of a 

 host of stars, but of incandescent gas; and the more 

 matured views of Sir William Herschel were estab- 

 lished upon a sound scientific basis. 



During the four years following this observation 

 Huggins subjected the light from seventy other 

 nebulae to analysis ; and of them about one-third, 

 including the Great Nebula in Orion, proved to be 

 gaseous. The remaining two-thirds yielded " con- 



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