1880.] 



on the Photographic Spectra of the Stars. 



291 



which the predominant colour is less refrangible. These stars are 

 of a full red colour. 



Now we return to the class of white stars. [The original photo- 

 graphs were exhibited on the 

 screen.] As this photograph is a 

 negative, of course the black lines 

 are represented by transparent 

 spaces and the continuous spectrum 

 by a dark band. We shall be able, 

 therefore, better to study the peculi- 

 arities of the spectrum if we substi- 

 tute for it a positive taken by direct 

 superposition. Here (Fig. 8) the 

 dark and light are not reversed. 

 The circumstance, which is so 

 marked as to compel us to give it 

 first attention, is the distinctly sym- 

 metrical character of this strong 

 group of lines. When the negative 

 is examined under suitable condi- 

 tions of illumination, twelve lines 

 may be counted. As the refrangi- 

 bility increases, the lines diminish 

 in breadth and the distance be- 

 tween any two lines is less as the 

 refrangibility of the lines in- 

 creases. It is also of importance to 

 notice that the spectrum does not 

 end with them. Beyond the last of 

 the group of lines the continuous 

 spectrum runs on far beyond S in 

 the ultra-violet. The point where 

 the group ends is between M and N. 



The first in order of refrangi- 

 bility of these lines is the well- 

 known line of hydrogen near G, 

 which you saw in the visible spec- 

 trum of the star. The second of 

 these lines is also a line of hydro- 



gen; 



coincident with h of the solar 



spectrum. The next line coincides 

 in position with the strong line H 

 of the solar spectrum. But where 

 is H2 or K ? It is represented by 

 this very thin line, which is barely 

 recognziable. You remember how narrow a slit was used, and that if 

 this were a photograph of the solar spectrum, some seven lines or 

 more would be clearly visible in this space. AVe shall now be able to 



X 2 



