504 POPULAR SCIENCE MONTHLY. 



red, one in the bluish green, and the two others in the violet. All four 

 of these lines belong to hydrogen. Their marked peculiarity is their 

 breadth, which tends to show that the absorbing layer is of considerable 

 thickness or is subjected to a great pressure. Besides these broad rays, 

 fine metallic rays are found in the brighter stars of this type. Secchi 

 considers that this is the most numerous type of all, half the stars which 

 he studied belonging to it. 



The second type is that of the somewhat yellow stars, like Capella, 

 Pollux, Arcturus, Procyon, etc. The most striking feature of the spec- 

 trum of these stars is its resemblance to that of our sun. Like the latter, 

 it is crossed by very fine and close black rays. It would seem that the 

 more the star inclines toward red, the broader these rays become and 

 the easier it is to distinguish them. We give a figure showing the 

 remarkable agreement between the spectrum of Capella, which may be 

 taken as an example of the type, and that of the sun. 



The spectra of the third type, belonging mostly to the red stars, are 

 composed of a double system of nebulous bands and dark lines. The lat- 

 ter are fundamentally the same as in the second type, the broad nebulous 

 bands being an addition to the spectrum, en Herculis may be taken as 

 an example of this type. 



It is to be remarked that, in these progressive types, the brilliancy 



;'. 



Fig. 7. Spectrum with both Bright and Dark Lines. 



of the more refrangible end of the spectrum continually diminishes 

 relatively to that of the red end. To this is due the gradations of color 

 in the stars. 



To these three types Secchi subsequently added a fourth, given by 

 comparatively few stars of a deep red color. The spectra of this class 

 consist principally of three bright bands, which are separated by dark 

 intervals. The brightest is in the green; a very faint one is in the blue; 

 the third is in the yellow and red, and is divided up into a number of 

 others. 



To these types a fifth was subsequently added by Wolf and Eayet, of 

 the Paris Observatory. The spectra of this class show a singular mix- 

 ture of bright lines and dark bands, as if three different spectra were 

 combined, one continuous, one an absorption spectrum, and one an 

 emission spectrum from glowing gas. Less than a hundred stars of this 

 type have been discovered. A very remarkable peculiarity, which we 



