112 SPECIAL RESULTS EN THE URANOLOGICAL PORTION 



between the violet and the red rays. Ptolemy, in his cata- 

 logue of the fixed stars, calls six stars vxoKippot, fiery red, ( 212 ) 

 viz. Arcturus, Aldebaran, Pollux, Antares, Betelgueze, and 

 Sirius. Cleomedese ven compares Antares with the red hue ( 213 ) 

 of Mars, which is itself called sometimes wppos, and some- 

 times 7TVpOl(ti)Q. 



Of the six stars above enumerated, five have still, in our 

 days, a red or reddish light : Pollux is still marked in our cata- 

 logues as reddish, whereas Castor is said to be greenish. ( 214 ) 

 Sirius, on the other hand, offers the solitary example of an 

 historically -proved alteration of colour, for it has at present 

 a perfectly white light. Some great revolution of nature ( 215 ) 

 must doubtless have taken place on the surface or in the pho- 

 tosphere of such a fixed star, such a distant sun, as already 

 Aristarchus of Sarnos would have called the fixed stars, to 

 disturb the process by which, through the absorption of other 

 complementary rays (whether in the photosphere of the star 

 itself, or in wandering cosmical clouds), the less refrangible 

 red rays had once been the prevailing ones. Now that, from 

 the modern advances in optics, this subject has become one 

 of great and lively interest, it would be very desirable to be 

 able to assign some fixed limits in point of time to the epoch 

 of such an event in Nature as that of the disappearance of 

 the red colour of Sirius. In the time of Tycho Brahe the 

 light of Sirius must certainly have been already white; for 

 when the new star which appeared in Cassiopeia in 1572, and 

 which was at first of dazzling whiteness, was seen to assume, 

 in March 1 573, a red tinge, and, in January 1574, to become 

 once more white, the astonished observers who witnessed 

 these changes compared the red star to Mars and Aldebaran, 

 but not to Sirius. Perhaps Sedillot, or other philologists 



