154 SPECIAL RESULTS IN THE URANOLOGICAL PORTION 



forty-two years later, by a Professor of Praneker, Johann 

 Phocylides Holwarda. This discovery was followed in the 

 same century by that of two other variable stars : /3 Persei 

 (1669), described by Montanari, and x Cygrii (1687), de- 

 scribed by Kirch. 



The increased number of stars of this class which have 

 been observed since the beginning of the present century, 

 and the irregularities which have been remarked in their 

 periods, have excited in the highest degree the interest which 

 is taken in this very complicated group of phsenomena. 

 Erom the difficulty of the subject, and my earnest desire that 

 in this work the numerical elements, as the most important 

 fruit of all observation, should be given as they are afforded 

 by the most recent investigation, and according to the actual 

 state of our knowledge, I have requested the kind aid of 

 the astronomer who, among our cotemporaries, has devoted 

 himself with the greatest activity and the most brilliant suc- 

 cess to the study of periodically varying stars. I laid before 

 my kind friend Argelander, Director of the Astronomical 

 Observatory at Bonn, in the fullest confidence, the doubts 

 and questions to which my own inquiries had given occa- 

 sion ; and I am indebted solely to his manuscript commu- 

 nications for what follows, great part of which has not yet 

 been otherwise published. 



The greater number of variable stars are red or reddish, 

 but by no means all. So, for example, besides Persei 

 (Algol in the head of Medusa), Lyrse and e Aurigae have 

 also white light. 77 Aquilse is somewhat yellowish; and so, 

 in a still less degree, is Geminorum. The statement for- 

 merly made, that some variable stars, and particularly Mira 

 Ceti, were redder while their brightness was diminishing than 



