NATURAL SCIENCES OP PHILADELPHIA. 51 



On report of the respective committees, the following were ordered 

 to be published : 



Additions to the Catalogue of STARS which have Changed their Colors ; or 

 which have appeared with different Colors at different times.* 



BY JACOB ENNIS. 



Sirius. On the re-appearance of this star during the months of November 

 and December, 1863, I very carefully watched its color, Its decided change of 

 appearance since the early part of the year greatly interested me. Instead of 

 a full bright green, it was of a pale yellowish green. During those two months 

 the atmosphere passed through the extremes of variability, but the pale yellow- 

 ish green of Sirius remained constant. On the 29th of December I asked Dr. 

 Wilcocks, the discoverer of its being purple three years ago, how Sirius ap- 

 peared to him now? Without knowing my opinion, and without any hesita- 

 tion, he answered, " It is not as green as it was when visible last winter." 

 This coincidence of his views with my own confirms the idea that he was not 

 mistaken three years ago, as he since supposed, when he regarded the color of 

 Sirius to be violet. According to these evidences this great star has had five 

 distinct colors : red during the ancient times ; white in 1850, and subsequently ; 

 violet in 1861 ; full green in the autum and winter of 1862 3 ; pale yellowish 

 green in November and December of 1863. 



Note of April 19th, 1864. The above observations were written early in 

 January. For the last three or four weeks the green color of Sirius has again 

 been tinged with blue; but this evening no blue is visible, and the yellow 

 mingled with the green is conspicuous. In this I am confirmed by a member 

 of the Academy whose powers of observation are remarkably good. The clear- 

 ness of the atmosphere is perfect, but the moon is large and bright, and 1 have 

 a suspicion that the yellow of Sirius, though in a different quarter of the 

 heavens, may be due to the moon's reflected rays. The following portions of 

 this paper, except that on No. 21, were written previous to the 10th of Novem- 

 ber, 1863, at which date they were presented. 



Altair and Deneb, or Alpha Cygni. The former of these 6tars was de- 

 scribed by Humboldt in 1850 as yellow and the latter as white. They were 

 numbered 18 and 19 in this Catalogue, and announced to be blue in June, 

 1863. I had watched them for several months nearly every clear night, and, 

 on the 20th of August, I first noticed that they were green. On all good nights 

 since then they have appeared to myself and to others, whose opinions 1 have 

 solicited, to be conspicuously green ; but on damp, slightly hay nights, from 

 the effects of the atmosphere, they appear blue. It is rather oppressive for 

 me to make this announcement, for I have been obliged already, in a former 

 communication, to say that two other large stars, Sirius and Vega, had changed 

 from blue to green. Certainly this change does not arise from any peculiari- 

 ties of vision, for I have in all cases carefully consulted the views of others 

 and found them to accord with my own. I know not how to attribute the 

 change to atmospheric causes, for I had observed them all, except Sirius, to be 

 blue several months before, in all weathers. 



Vega. For the last four or five weeks, thi3 star has not appeared to me as 

 green as during last summer. Its rather bluish appearance may, perhaps, be 

 attributed to greater haziness of the atmosphere. 



Castor. See No. 10 of this Catalogue. In addition to the colors already 

 given, the two companions of this double star have been described as follows . 

 Yellow and yellowish by Sestini ; greenish yellow and green by Dembowski ; 



* See these Proceedings for 1S63, pp. 26.96, 159. 



1864.] 



