Mr. J. N. Lockyer. On the Classification [Apr. 12, 



have so far studied that the star would continue for a considerable 

 time at its minimnm. 



Broadly speaking, then, we may say that the variables in this group 

 are close doubles. The invisibility of the companion being due to 

 the nearness to the primary or to its faintness. 



Double Stars. 



If, in connexion with this subject, we refer to the various observa- 

 tions which have been made of double nebulae and stars, we are 

 driven to the conclusion that in many cases a double star has at one 

 time existed as a double nebula, while on the other hand, from what 

 has been stated it seems probable that in many cases the companion is a 

 late addition to the system. It would seem as if we may be able in 

 the future, by observing the spectra of double stars, or possibly even 

 their colours when once each particular colour has been attached to 

 a particular spectrum, to discriminate between these two condi- 

 tions. 



In discussing this matter, however, a difficulty arises on account oE 

 the fact that on the new view there will be no constant relation 

 between the mass of a swarm and its brightness. When we see a 

 ''star '' of a certain magnitude, we cannot tell from its brightness 

 alone whether it is a large faint one or a small bright one, for a large 

 body at a low temperature may be equalled or even excelled in bright- 

 ness by a smaller " star " at a higher temperature. But when we know 

 the spectra of the bodies, we also sknow their relative temperatures. 

 In the absence of spectroscopic details, colour helps us to a certain 

 extent. 



If a pair of " stars " of unequal masses have condensed from the same 

 nebulosity, the smaller one will be further advanced along the tempe- 

 rature curve than the larger one, and the colours and spectra will be 

 different ; but it is not imperative that the magnitudes shall be unequal, 

 for the smaller swarm will for a time be considerably hotter than the 

 larger one. 



If the masses be very unequal, the smaller one will have the smaller 

 magnitude for the longest time. Where there is a great difference in 

 magnitude, therefore, it is generally fair to assume that the one with 

 the smaller magnitude has also the smaller mass. 



Another difficulty in the discussion, in the absence of spectroscopic 

 details, is due to the similarity in colour of bodies at opposite points 

 of the temperature curve. Thus, bodies in Group III have, as far as 

 we at present know, exactly the same colour, namely, yellow, as those 

 in Group V. Again, many of the members of Group II have the same 

 colour as some in Group VI. 



The general conditions with regard to this subject may be thus 

 briefly stated :• — If the magnitudes, colours, and spectra of the two 



