208 A CENTURY'S PROGRESS IN ASTRONOMY. 



nebula was not a star-cluster, but a mass of glow- 

 ing gas, hydrogen, and some other unknown 

 substance, now named "nebulium." By 1868 

 Huggins had observed the spectra of seventy 

 nebulse. Of these one - third proved to be 

 gaseous, among them the great Orion nebula 

 which Lord Rosse was believed to have resolved 

 into stars. In the spectrum of the latter, the 

 " chief nebular line " was at first ascribed by 

 Huggins to nitrogen, but this was a mistake. 

 Later, it was believed by Lockyer to coincide 

 with the fluting of magnesium, but this was 

 disproved by Huggins in 1889-90, and by 

 Keeler in 1890-91. The great nebula in 

 Andromeda and the great spiral in Canes 

 Venatici were found by Huggins to display a 

 continuous spectrum, and a similar discovery 

 was made in regard to the cluster M 13 in 

 Hercules, and other star-clusters. In the case 

 of the nebulae, it is not believed that the con- 

 tinuous spectrum is due to the existence of sun- 

 like bodies, as a gas under pressure would give 

 a continuous spectrum. 



The Orion nebula has been more thoroughly 

 studied than any other object of its class. The 

 application of photography to spectroscopy has 

 done much to further the study of the lines in 

 the nebular spectrum. In 1886 Copeland de- 



