224 COSMOS. 



(as being the most important result of all observations), so 

 far as in the present state of the science they have been 

 ascertained, I have availed myself of the friendly aid of that 

 astronomer who of all our contemporaries has devoted him- 

 self with the greatest diligence, and with the most brilliant 

 success, to the study of the periodically varying stars. The 

 doubts and questions called forth by my own labours I con- 

 fidently laid before my worthy friend Argelander, the director 

 of the Observatory at Bonn ; and it is to his manuscript com- 

 munications that I am solely indebted for all that follows, 

 which for the most part has never before been published. 



The greater number of the variable stars, although not 

 all, are of a red or reddish colour. Thus, for instance, 

 besides Persei (Algol in the head of Medusa), ft Lyrje 

 ind Auriga3 have also a white light. The star rj Aquilae 

 is rather yellowish ; so also in a still less degree is Gemi- 

 norum. The old assertion that some variable stars (and 

 especially Mira Ceti) are redder when their brilliancy is 

 on the wane than on the increase, seems to be groundless. 

 Whether in the double star a Herculis (in which, according to 

 Sir John Herschel, the greater star is red, but according to 

 Struve yellow, while its companion is said to be dark blue) 

 the small companion, estimated at between the 5th to the 

 7th magnitude, is itself also variable, appears very pro- 

 blematical. Struve 8 himself merely says. Suspicor minorem 

 esse variabilem. Variability is by no means a necessary 



8 Compare Madler, Astr., s. 438, note 12, with Struve 

 Stellarum compos, mensurce microm. pp. 97 and 98, star 2140. 

 " I believe," says Argelander, " it is extremely difficult with 

 a telescope having a great power of illumination to estimate 

 rightly the brightness of two such different stars as the 

 two components of a Herculis. My experience is strongly 

 against the variability of the companion ; for during my many 

 observations in the day-time with the telescopes of the 



