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



passing from Group II to Group III ; and what other phenomena 

 accompanied the transition, and what were the special phenomena 

 which accompanied the gradual distension of the hydrogen lines in 

 passing to 'Group IV. There has not been a sufficient number of fine 

 nights since the work commenced to enable this to be done com- 

 pletely. 



II. General Statement of Conditions. 



A general statement of the conditions of the problem was given in 

 the Bakerian Lecture (p. *26); and I here reproduce the greater part 

 of what I then wrote on the subject. 



" The passage from the second group to the third brings us to those 

 bodies which are increasing their temperature, in which carbon radia- 

 tion and fluting absorption have given place to line absorption. At 

 present the data already accumulated by other observers have not 

 been discussed in such a way as to enable us to state very definitely 

 the exact retreat of the absorption by which I mean the exact order 

 in which the absorption lines fade out from the first members to the 

 last in the group. We know generally that the earlier bodies will 

 contain the line absorption of those substances of which we get a 

 paramount fluting absorption in the prior group. We also know 

 generally that the absorption of hydrogen will increase while the 

 other diminishes. 



" The next group the Fourth brings us to the stage of highest 

 temperature, to stars like a Lyras, and the division between this group 

 and the prior one must be more or less arbitrary, and cannot at present 

 be defined. One thing, however, is quite clear, that no celestial body 

 without all the ultra-violet lines of hydrogen discovered by Dr. 

 Huggins can claim to belong to it. 



" We have now arrived at the culminating point of temperature, 

 and next pass to the descending arm of the curve. The Fifth Group, 

 therefore, will contain those bodies in which the hydrogen lines begin 

 to decrease in intensity, and other absorptions to take place in conse- 

 quence of reduction of temperature. 



" It seems fair to assume that physical and chemical combinations 

 will now have an opportunity of taking place, thereby changing the 

 constituents of the atmosphere; that at first, with every decrease of 

 temperature and increase in the absorption, lines may be expected, 

 but it will be unlikely that the coolest bodies in this group will 

 resemble the coolest bodies in Group III. 



" Up to the present time observers have not recognised the import- 

 ance of these considerations, and since only one line of temperature, 

 and that a descending one, has been considered, no efforts have been 

 made to establish the necessary criteria between Groups III and V." 



It follows from the above that criteria are only possible from the 



