OF THE COSMOS. PERIODICALLY VARIABLE STARS. 155 



while it was increasing, appears unfounded. Whether in the 

 double star a Herculis, in which Sir William Hershchel calls 

 the large star red, and Struve calls it yellow and its compa- 

 nion dark-blue, this small companion which is estimated from 

 the 5th to the 7th magnitude, be itself also variable, appears 

 very problematical. Struve ( 274 ) himself says only "suspicor 

 minorem esse variabilem." Variability is by no means 

 attached to redness of colour. There are many red, and 

 some very red, stars, as Arcturus and Aldebaran, in which, 

 hitherto, no variation has ever been observed ; and the ex- 

 istence of any variability in a star in Cepheus (No. 7582 of 

 the Catalogue of the British Association), which, on account 

 of its extraordinary redness, was called by William Herschel, 

 in 1782, the Garnet is more than doubtful. 



It is difficult to say exactly what ought to be regarded 

 as the whole known number of periodically variable stars, 

 because the periods which have already been deduced are of 

 very unequal degrees of certainty. The two variable stars 

 in Pegasus, as well Hydrse, e Aurigas, and a Cassiopeise, 

 have not the same certainty as Mira Ceti, Algol, and 

 Cephei. In drawing up a table, therefore, the question 

 arises, what degree of certainty is to be regarded as suffi- 

 cient. As will be seen in the general table at the close of 

 this investigation, Argelander reckons the number of satis- 

 factorily determined periods at only 24. ( 275 ) 



We have seen that the phsenomenon of variability belongs 

 to some white stars as well as to red ones, and it is also 

 found to exist in stars of very different magnitudes : for 

 example, in one star of the 1st magnitude, a Orionis; in 

 Mira Ceti, a Hydrse, a Cassiopeise, and # Pegasi, all of the 

 2nd magnitude ; Persei, 2' 3 magnitude ; and in rj Aquilse 

 and Lyrse, 3*4 magnitude. There are also, and in much 



