446 



Mr. J. Norman Lockyer. 



with the sun in his Class Ila, and more recently Dr. Scheiner has 

 placed another star of the group, a Persei, between a Cygni and the 

 sun. It is right to add that Drs. Yogel and Scheiner differ very 

 considerably as to the classification of a Cygni, and this still further 

 complicates matters. 



It will be seen that the question between the two classifications is 

 a very sharp one. Is the difference between stars like d Cephei 

 and stars resembling the sun solely due to a temperature differ- 

 ence, as it is on Vogel's view, or does it in part represent, as I 

 contend, a physical difference between the two classes, since on my 

 view stars like 8 Cephei consist of uncondensed swarms of meteorites 

 of increasing temperature, while those like the sun are masses of 

 vapour in which there are photospheres and relatively quiet atmo- 

 spheres ? 



Since the work recently communicated to the Royal Society 

 enables me to go still further forward, I propose to discuss in the 

 present paper the points relating to the classification of stars of the 

 S Cephei class. 



In Vogel's classification all stars are regarded as cooling bodies, 

 while one of the chief points of mine is the distinction between stars 

 which are getting hotter and those which are becoming cooler. 

 Thus, while stars like £ Cephei and those like the sun are grouped 

 together by Vogel in his Class Ila, they are in mine divided into two 

 groups, Group III including 8 Cephei and Group V the sun. 



When I first suggested the new classification I fully recognised 

 the difficulty of separating Groups III and V. Thus I wrote in 

 1888:— 



" With our present knowledge, it is very difficult to separate those 

 stars the grouping of which is determined by line absorption into 

 Groups III and V, for the reason that so far, seeing that only 

 one line of temperature, and that a descending one, has been con- 

 sidered, no efforts have been made to establish the necessary 

 criteria."* 



In the following year I gave the results of some visual observa- 

 tions of stellar spectra which seemed to justify the separation of the 

 stars with line spectra into two groups, and to suggest the necessary 

 criteria for distinguishing them.f 



Soon after this a photographic investigation of stellar spectra was 

 commenced at Kensington, and as a result of that work the conclu- 

 sions at which I had arrived from a discussion of the eye observations 

 were confirmed. The additional details shown by the photographs 

 enabled me to extend the classification given in 18884 



* ' Eoy. Soc. Proc.,' vol. 44, p. 27. 



t ' Hoy. Soc. Proc.,' vol. 45, p. 380. 



X ' Phil. Trans.,' A, vol. 184, p. 125 (1893). 



