178 COSMOS. 



rays, either in the photosphere of the star itself, or in the 

 moving cosmical clouds by which it is surrounded. It is to 

 be wished that the epoch of the disappearance of the red 

 colour of Sirius had been recorded by a definite reference to the 

 time, as this subject has excited a vivid interest in the minds 

 of astronomers since the great advance made in modern 

 optics. At the time of Tycho Brahe the light of Sirius was un- 

 doubtedly already white, for when the new star which appeared 

 in Cassiopeia, in 1572, was observed in the month of March, 

 1573, to change from its previous dazzling white colour to a 

 reddish hue, and again became white in January, 1574, the red 

 appearance of the star was compared to the colour of Mars and 

 Aldebaran, but not to that of Sirius. M. Sedillot, or other phi- 

 lologists conversant with Arabic and Persian astronomy, may 

 perhaps some day succeed in discovering evidence of the 

 earlier colour of Sirius, in the periods intervening from El- 

 Batani ( Albategnius) and El-Fergani (Alfraganus) to Abdur- 

 rahman Sufi and Ebn-Junis (that is, from 880 to 1007), and 

 from Ebn-Junis to Nassir-Eddin and Ulugh-Beg (from 1007 

 to 1437). 



El-Fergani (properly Mohammed Ebn-Kethir El-Fergani), 

 who conducted astronomical observations in the middle of the 

 tenth century at Rakka (Aracte) on the Euphrates, indicates 

 as red stars (stella rujfce of the old Latin translation of 1590) 

 Aldebaran, and, singularly enough, 90 Capella, which is now 

 yellow and has scarcely a tinge of red, but he does not men- 

 tion Sirius. If at this period Sirius had been no longer red, 

 it would certainly be a striking fact that El-Fergani, who 

 invariably follows Ptolemy, should not here indicate the 



60 In Muhamedis Alfragani cJironologica et astronomica 

 Elementa, ed. Jacobus Christmannus, 1590, cap. 22, p. 97, 

 we read: "Stella ruffa in Tauro Aldebaran; stella ruffa in 

 Oeminis quse appellatur Hajok, hoc est Capra." Alhajoc, 

 Aijuk are, however, the ordinary D.ames for Capella Aurigse, 



