188 



Sir Norman Lockyer. 



Stars of intermediate Temperature. 



Ascending Series. 



/3 Crucis. 

 £ Tauri. 

 Kigel. 

 a Cygni. 



[ ] 

 Polaris. 



Aldebaran. 



Descending Series. 



Achernar. 



Algol. 



Markab. 



[ ] 



Sirius. 



Procyon. 



Arcturus. 



Stars of lowest Temperature. 



Ascending Series. 



Catalogue of 



Antares, one of the brightest 

 stars in Duner's 

 Class Ilia* 



[Nebulae. 



Descending Series. 



19 Piscium, one of the brightest 

 stars in Dnner's Catalogue of 

 Class III5. 



[Dark Stars.] 



In order to make quite clear that both an ascending and a descend- 

 ing series must be taken into account, I give herewith (Plate 7) two 

 photographs showing the phenomena observed on both sides of the 

 temperature curve in reversing layers of stars of nearly equal mean 

 temperatures, as determined by the enhanced lines. The stars in 

 question are : — 



Sirius (descending). \ 

 ling). J 



a Cygni (ascending). 



Procyon (descending 

 y Cygni (ascending). 



The main differences to which I wish to draw attention are the very 

 different intensities of the hydrogen lines in Sirius and a Cygni, and the 

 difference in the width and intensities of the proto-metallic and metallic 

 lines in Procyon and y Cygni. These differences, so significant from a 

 classification point of view, were first indicated in a communication to 

 the Society in 18871, and the progress of the work on these lines has 

 shown how important they are. I have based the group — or generic — 

 words upon the following considerations. 



As we now know beyond all question that a series of geological 

 strata from the most ancient to the most recent brings us in presence 

 of different organic forms, of which the most recent are the most com- 



* 1 Sur les Etoiles a spectres de la troisieme classe.' 

 f ' Eoy. Soe. Proc., 5 vol. 43, p. 145. 



