384 Mr. J. N. Lockyer. [Feb. 28, 



account of the increased temperature. Finally, the spectrum will be 

 of the type represented by a Lyrae. 



The question here arises, where are we to draw the line between 

 Group II and Group III ? If my definition of Group II as the 

 " mixed fluting " group be accepted, we must obviously draw the line 

 at the stage where carbon radiation disappears. The iron fluting at 

 615 remains for a considerable time after this happens, so that the 

 earliest species of Group III will be marked by the absorption 

 fluting of iron in addition to the characteristic line absorption. This 

 being the case, observations show that Aldebaran is a good example 

 of an early stage. 



IV. The Relations of the Later Species of Group III to /Stars of 



Group IV. 



The spectrum characteristic of Group IV is that of excessive 

 hydrogen absorption, with other lines exceedingly faint. In passing 

 from. Group III to Group IV, therefore, the hydrogen lines must 

 thicken whilst the metallic lines thin. In a letter to M. Dumas in 

 1872 I suggested that possibly the simplification of the spectrum of 

 a star might be associated with the highest temperature of the vapour, 

 and that idea seems to have been accepted by other investigators 

 since that time. It is now generally accepted that stars with thick 

 hydrogen lines (Group IV) are the hottest stars. 



The reason why we have hydrogen absorption in such great excess, 

 is, I have little doubt, that most other substances have been dis- 

 sociated by the intense heat resulting from the condensation of the 

 meteoric swarm. We are, in fact, driven to this conclusion, because 

 the hydrogen which was originally occluded by the meteorites must 

 have been driven off long before this temperature was reached. 



In passing from a star like <x Tauri to one like a Lyrae, the metallic 

 lines would thin and disappear in some order determined, by their dis- 

 sociability or some other quality. The later stars of Group III are 

 therefore very closely related to stars of Group IV, and the division 

 between the two must be more or less arbitrary. For simplicity's 

 sake, I have taken Group IV as the point of maximum temperature. 



V. The Observations having reference to Specific Differences in 

 Group III. 



The observations have been made at the Astronomical Laboratory 

 at South Kensington by Mr. Fowler, assisted by Messrs. Baxandall 

 and Coppen (with the 10-inch equatorial and star spectroscope by 

 Hilger) in connexion with my own observations at Westgate (made 

 with a 12-inch mirror, kindly lent to me by Mr. Common, and a small 

 Maclean spectroscopic eyepiece). All measurements and comparisons 

 suggested by my own observations were made by my assistants, as at 



