498 HISTORY OF SCIENCE. 



brilliancy, had reverted nearly to its former condition of a star of the 

 tenth magnitude. The bright lines disappeared as the light of the star 

 waned. Other observers have announced that bright lines are visible 

 in the spectra of several very small stars. Father Secchi in 1869 pro- 

 posed to classify the stars in several groups according to the kind of 

 spectrum that each exhibited. One group consists of those stars 

 which give spectra intersected by many fine dark lines. Such are the 

 stars Pollux, Capella, and, in general, those stars which shine with a 

 yellow light. Our sun also is regarded as a star belonging to this 

 group. A second group comprises many of the brightest stars in the 

 heavens, namely, those which have a white lustre, and their spectra 

 are characterized by a small number of broad dark lines : such are 

 Sirius and Lyrce, etc. A third group includes reddish-coloured stars, 

 which give spectra having alternate dark and light bands or spaces : 

 such are Htrculis and Orionis and ft Pegasi. 



The study of spectra was greatly facilitated by the substitution of 

 photography for mapping in the delineation of the lines. In this way 

 the spectrum is made in fact to produce its own map, and it is found 

 that the " chemical rays " which accompany the luminous rays faith- 

 fully indicate on the prepared plate the position and intensities of all 

 the lines. Rutherford, of New York, appears to have been the first 

 to turn to account the power of photography in the delineation of 

 spectra. Mr. Huggins photographed the spectrum of Sirius. It is 

 marvellous that, from the inconceivable distance of 130 millions of 

 millions of miles, the light of Sirius, reaching us after a journey of 

 twenty-one years' duration, should bring with it the power of impressing 

 the sensitive photograph-plate that secures for us the records of the 

 chemical constitution of the star as it was twenty-one years before. 



Mr. Huggins was the first to direct the spectroscope to those mys- 

 terious bodies, the nebulae. He found that the light of some of the 

 nebulae was too faint to yield spectra distinct enough to give satis- 

 factory results. He was able to determine the characters of the 

 spectra of about seventy nebulas. Of these about one-third gave a 

 spectrum with bright lines, and differed one from another only by the 

 relative intensities of the lines and the degree in which the lines were 

 accompanied by a continuous spectrum. Bright line spectra are pro- 

 duced only by incandescent gases, so far as we know, and therefore 

 the inference is, that in these nebulae we have to do with gaseous 

 matter. The next point was to determine the nature of the gases. 

 One of these lines was exactly coincident with one of the hydrogen 

 lines, another with the bright of the nitrogen lines, but the third occu- 

 pied a position corresponding with no known element. Fig. 226 shows 

 one of these nebulae spectra, and the lines of hydrogen and of nitrogen 

 are placed beside it. It will be observed that the nitrogen spectrum 

 here shown has a double and not a single line at N. This is the 

 spectrum yielded by an electric spark passed through nitrogen in 



