PERIODICAL STARS. 165 



with uniform intensity. We shall first of all dwell exclu- 

 sively on the first kind of variability ; of this, the earliest in- 

 stance accurately observed is furnished (1638) by Mira, a 

 star in the neck of Cetus. The East-Friesland pastor, David 

 Fabricius (the father of the discoverer of the spots on the 

 sun), had certainly already observed this star on the 13th of 

 August, 1596, as of the third magnitude, and in October of 

 the same year he saw it disappear. But it was not until for- 

 ty-two years afterward that the alternating, recurring vari- 

 ability of its light, and its periodic changes, were discovered 

 by the Professor Johann Phocylides Holwarda, Professor of 

 Franeker. This discovery was further followed in the same 

 century by that of two other variable stars, (3 Persei (1669), 

 described by Montanari, and % Cygni (1687), by Kirch. 



The irregularities which have been noticed in the periods, 

 together with the additional number of stars of this class 

 which have been discovered, have, since the beginning of the 

 nineteenth century, awakened the most lively interest in this 

 complicated group of phenomena. From the difficulty of the 

 subject, and from my own wish to be able to set down in the 

 present work the nurwerical elements of this variability (as 

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

 in the present state of the science they have been ascertain- 

 ed, I have availed myself of the friendly aid of that astrono- 

 mer who of all our cotemporaries has devoted himself 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 labors I confidently 

 laid before my worthy friend Argelander, the director of the 

 Observatory at Bonn, and it is to his manuscript communi 

 cations 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 color. Thus, for instance, besides (3 

 Persei (Algol in the head of Medusa), (3 Lyrae and Aurigse 

 have also a white light. The star rj Aquilae is rather yellow- 

 ish ; so also, in a still less degree, is Geminorum. 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 fifth to the seventh magnitude, is 



