Double Stars. 379 



tion, as inferring the probability of actual change ; and those 

 who are interested in this curious subject will find this group 

 well worthy of study. My own impression has the strong 

 confirmation of Knott, who, in July, 1865, noted D (the star 

 in question) " white, with pale dash of blue. D is certainly 

 not so blue as 0." 



While speaking of coloured light, it may not be uninterest- 

 ing to note the comparative absence of any decided hue in 

 the larger stars of the Pleiades, as contrasted with many other 

 parts of the sky, for instance, the galaxy region of Sagitta, 

 where most of the leading stars show yellow, or ruddy light. 



This opportunity should be taken for a careful study of the 

 colours of the beautiful double star a Piscium (Int. Obs., hi., 

 55), as to which there is a remarkable discrepancy among 

 astronomers. I have never been able to satisfy myself as to 

 their real character. 



The possession of the beautifully- figured 94-inch " With }> 

 speculum, already alluded to, leads to the addition of the two 

 following objects as tests for those who may be equally fortu- 

 nate in optical means. 



168. S Gygni (^ 2579), at the end of the p arm of the 

 cross represented by the principal stars of that asterism. 

 1"*8. 25°-6. 3| and 9. Pale yellow and sea-green. Such 

 were Smyth's data for this celebrated but very difficult binary, 

 1842-56, which $ saw 2"-5 apart, 1783*72, "out single 1802, 

 1804, as did H. and South 1823, and South 1825. 2, how- 

 ever, found it r-91, 1826-55; Smyth l"-5, 1837-78- Dawes 

 l"-67, 1865-38; Knott 1"- 7, 1866-68; while its orbital revo- 

 lution has carried it through zero from the nf into the np 

 quadrant, from IjFs 71 0- 39, 1 783- 72, by a regular gradation to 

 Knott's 348°-31, 1866-68:— the " periastron" being con- 

 sidered to have occurred in 1860. This extremely difficult test 

 I have seen so fairly with 450 in only a moderately favourable 

 state of air, and at a comparatively low elevation, that it may 

 be inferred to be easy under really advantageous circum- 

 stances ; on the majority of nights it would, of course, be waste 

 of time to look for it. The other object is 



169. yu, Andromedce ; the 4th mag. star next np /3, in the 

 line pointing upwards to the Great Nebula. This has a com- 

 panion at 45" and 115°, 16 mag., which is consequently as 

 severe a test of light as the previous pair is of definition. 

 It is so minute that Smyth saw it but once with his 5-9-inch 

 object-glass, and when the larger star was hidden by a bar 

 in the field. 



This most delicate point I have caught up without much 

 trouble, and that while, from a mistake as to the angle, I had 

 so little expectation of seeing it where I found it, that I had 



