Photographic Spectra of Stars to the 3-J Magnitude. 215 



of Division I are in the first stage of stellar development from the 

 gaseous nebula?. Firstly, the correspondence of their absorption 

 spectra with the bright-line spectrum of the gaseous nebulae, as 

 shown by Campbell and others, is pointed out. Secondly, it is 

 shown that the photographic spectra of the groups of stars involved 

 in, or connected with, the nebulae in the Pleiades and the great 

 nebula in Orion are helium spectra of Division I. Thirdly, it is 

 shown that the helium stars of Division I, and the gaseous nebulae 

 are similarly distributed in the galactic zones. 



The special characteristics of Divisions I, II, and III are discrimi- 

 nated as follows : — Bright-line stars are classed in accordance with 

 their absorption spectra. 



Division T. — The Orion stars, or the helium type. The general 

 characteristics are the spectra of cleveite gas and hydrogen. The 

 K line of calcium is usually wanting. 



Subdivision (a). — There are, in addition, special lines, the source 

 of which is unknown. The possibility is suggested of these being 

 due to oxygen. Thalen's spark spectrum of oxygen is collated with 

 the stellar spectra: 



Subdivision (6). — There are other special lines attributed to 

 calcium, barium, and magnesium. These lines are persistent in sub- 

 sequent divisions, and mark the order of transition from Division I to 

 Division II. The helium lines gradually disappear. 



Division II. — Sirian stars, or the hydrogen type. The character- 

 istics are : — Helium wanting — Very strong hydrogen — The K line 

 usually sharply defined — A delicate line spectrum due to calcium, 

 titanium, and faintly to iron — a Cygni belongs to this type, and 

 shows the characteristic line spectrum exceptionally distinct — The 

 ultra-violet spectrum is very strong. 



Division III. — Procyon stars, or the hydrogen - iron type. 

 Characteristics : — Strong hydrogen — Strong K — Usually a full iron 

 spectrum — The violet spectrum still strong, and the red weak — The 

 iron spectrum is collated with the photograph. This division com- 

 pletes the requisite subdivision of Secchi's Type I. 



Divisions IV, V, and VI are equivalent to Secchi's Types II, III, 

 and IV. Division IV is the solar type, and on account of our 

 knowledge of the solar spectrum it forms the basis of sidereal 

 spectroscopy. 



The block of space explored by means of the photographs is n 

 diameter about seven times the mean distance of first magnitude 

 stars from the sun. This is equivalent to about 255 light-years. 



The following is the table of distribution of the stellar spectra 

 here given. The distribution of the gaseous nebulae, planetary and 

 extended, has been added. 



