ASTKONOMY. Ill 



some further more or less probable assumptioDs) deduces au expres- 

 siou for the maguitude of the cooling star at any time. lu order to 

 compare tbis formula with Herr Miiller's photometric measures of the 

 Nova, extending from 1885, September 2, to October 13, Professor Seel- 

 iger assumes that «=1, and that the epoch for which the time ^=0, 

 is 1885, August 27, 8^^ Berlin mean time. Using quite approximate 

 values of the constants involved in his formula, it appears that there is 

 a good general agreement (the mean discordance being O.ll of a stelhir 

 magnitude) between the computed and observed values. The computed 

 magnitude corresponding to the epoch for which t=0, is 7.73. The fair 

 agreement shown by this comparison induces Professor Seeliger to 

 thii]k that the form of the expression which he has deduced is such as 

 would accurately represent the observations, provided that it were pos- 

 sible to determine the necessary constants with sufficient precision. 

 And as there is evidence to show that the nebula in Andromeda is, 

 l)artly at least, <;omposed of a vast number of faint stars, it appears, in 

 Professor Seeliger's opinion, not unreasonable to suppose that a collis- 

 ion was the cause of the sudden development of heat and light which 

 revealed itself to us as the appearance of a " new " star. 



With reference to the i)oint thus raised by Professor Seeliger, Berr 

 Auwers points out (Astron.Nachr., No. 2715) that the great simihirity of 

 the outburst in Andromedain 1885 to the phenomenon observed by him 

 in 1860 in the cluster 80 Messier in Scorpio is a strong confirmation of 

 Professor Seeliger's views. The probability that two variable stars of 

 such exceptional character should be i)rojected, in one case on a close 

 star-cluster, in the other case on an object which appears to be, in 

 part at least, a close star-cluster, is so small that it is almost necessary 

 to refer these outbursts to physical changes in the nebula? in which they 

 respectively appeared. (Observatory, April, 1886.) 



Dr. Mills (Nature 33 : 440) in criticising Professor Seeliger's collision 

 hy[)othesis suggests that the blazing out of the Nova may be merely a 

 physico-chemical consequence of cooling; and it has been pointed out 

 by Mr. Castell-Evaus (Nature, 33: 486) that practically the same expla- 

 nation was suggested in 1878 by Prof. E, Meldola in a paper published 

 in the Philosophical Magazine for July of that year. Professor Mel- 

 dola says: " It is conceivable that in certain cases the composition of 

 a star's atmosphere maybe such as to permit a consideiable amount of 

 cooling before any combination takes place among its constituents ; un- 

 der such circumstances a sudden catastrophe might mark the period of 

 combination, and a star of feeble light would blaze forth suddenly, as 

 occurred in 1866 to r Coronse Borealis. In other cases, again, it is 

 possible that the composition of a star's atmosphere may be of such a 

 nature as to lead to a state of periodically unstable chemical equilib- 

 rium ; that is to say, during a certain period combination may b(- going 

 ou with the accompanying evolutiou of heat, till at leugth dissociatiou 



