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THE POPULAR SCIENCE MONTHLY. 



its physical condition. If the spectrum be a continuous one, consisting 

 of rays of every color or degree of refrangibility, then the source of light 

 is either a solid or liquid incandescent body ; if, on the contrary, the 

 spectrum be composed of bright lines only, then it is certain that the 

 light comes from luminous gas / finally, if the spectrum be continu- 

 ous, but crossed by dark lines interrupting the colors, it is an indica- 

 tion that the source of light is a solid or liquid incandescent body, but 

 that the light has passed through an atmosphere of vapors at a lower 

 temperature, which by their selective absorptive power have abstracted 

 those colored rays which they would have emitted had they been self- 

 luminous. 



Fig. 18. 



Spectrum of Nebula. 1 



When Huggins first directed his telescope in August, 1864, to one 

 of these objects, a small but very bright nebula, he found, to his great 

 surprise, that the spectrum, instead of being a continuous colored 

 band, such as that given by a star, consisted only of three bright lines. 



This one observation was sufficient to solve the long-vexed ques- 

 tion, at least for this particular nebula, and to prove that it is not a 

 cluster of individual, separable stars, but is actually a gaseous nebula, 

 a body of luminous gas. In fact, such a spectrum could only be pro- 



Fio. 19. 



Speotbum of Nebula compared with the Sun and some Teebestkial Elements. 



duced by a substance in a state of gas ; the light of this nebula, there- 

 fore, was emitted neither by solid nor liquid incandescent matter, nor 

 by gases in a state of extreme density, as may be the case in the sun 

 and stars, but by luminous gas in a highly-rarefied condition. 



In order to discover the chemical nature of this gas, Huggins fol- 

 lowed the usual methods of comparison, and tested the spectrum with 



1 From Herschel's Catalogue, No. 4,374. 



