80 



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



[Received April 5, 1888.] 



Paet V. — On the Cause of Variation in the Light op Bodies of 

 Groups I and II. 



I. General Views on Variability. 



In my former paper I referred to the collision of meteor-swarms as 

 producing "new stars," and to the periastron passage of one swarm 

 through another as prodnciug the more or less regular variability 

 observed in the case of some stars of the group under consideration. 



I propose now to consider this question of variability at somewhat 

 greater length, but only that part of it which touches non-condensed 

 swarms ; i.e., I shall for the present leave the phenomena of new stars, 

 and of those whose variability is caused by eclipses, aside. 



It is not necessary that I should pause here to state at length the 

 causes of stellar variability which have been suggested from time to 

 time. It will suffice, perhaps, that I should refer to one of the first 

 suggestions which we owe to Sir I. Newton, and to the last general 

 discussion of the matter, which we owe to Zollner (' Photometrische 

 Untersuchungen,' 76 and 77, p. 252). 



Newton ascribed that special class of variability, to which I 

 shall have most to refer in the sequel, as due to the appulse of 

 comets. 



" Sic etiam stellae fixae, qua? paulatim expirant in lucem et vapores, 

 cometis in ipsas incidentibus refici possunt, et novo alimento accensce pro 

 stellis novis haberi. Hujus generis sunt stellae fixae, quae subito appa- 

 rent, et sub initio quam maxime splendent, et snbinde paulatim evan- 

 escunt.. Talis fuit stella in cathedra Cassiopeiae quam Cornelius 

 Gemma octavo Novembris 1572 lustrando illam coeli partem nocte 

 serena minime vidit ; at nocte proxima (Novem. 9) vidit fixis omnibus 

 splendidiorem, et luce sua vix cedentem Veneri. Hanc Tycho 

 Brahaeus vidit undecimo ejus d em mensis ubi maxime splenduit ; et ex 

 eo tempore paulatim decrescentem et spatio mensium sexdecim evan- 

 escentem observavit " (' Principia,' p. 525, Glasgow, 1871). 



With regard to another class of variables he makes a suggestion 

 which has generally been accepted since : — 



" Sed fixae, quae per vices apparent et evanescunt, quaeque paulatim 

 crescunt, et luce sua fixas tertiae magnitudinis vix unquam superant, 

 videntur esse generis alterius, et revolvendo partem lucidam et partem 

 obscuram per vices ostendere. Vapores autem, qui ex sole et stellis 

 fixis et caudis cometarum oriuntur, incidere possunt per gravitatem 

 suam in atmosphaeras planetarum et ibi condensari et converti in 

 aquam et spiritus humidos, et subinde per lentum calorem in sales et 

 sulphura et tincturas et limum et lutum et argillam et arenam et 

 lapides et coralla et substantias alias terrestres paulatim migrare." 



