400 PROFESSOR C. PIAZZI SMYTH ON THE 



Struve's occurred in . . . • . 1832-53 



Sestini's occurred in . . . . . 1844-5 



My own occurred in . . . . . 18 56 58 



or, a period of twelve years, exact almost to the tenth of a year, separates each 

 of these apparent anomalies. Unless then some most extraordinary combination 

 of errors has occmTed, we have stumbled here on a perfectly unexampled case of 

 periodical change of colour in two stars, and a change which, in the course of 

 its cycle, goes through most conspicuous phases. 



So far as I know, this conclusion for the stars is quite new ; Struve, did 

 indeed, in his note at p. 98 of the " Mensurse Micrometricae," remark something 

 entirely unusual affecting these stars, and distinguishing them from all others 

 that he ever observed ; and I cannot but think that he meant to allude to some- 

 thing more than the mere average diversity of their colours one from the other, 

 for that is not an unusual feature amongst double-stars ; but what it may have 

 been, does not appear.* At all events he does not make, and at that period had 

 not observations to give him any hint or clue to a periodic time of change. 



In Position and Distance, something is also determinable from our observa- 

 tions : for even so late as 1860, the " Spec. Hart.," a high authority, speaks of the 

 established " fixity of this optical pair." Yet a glance at our collected table of re- 

 sults, beginning with Sir William Herschel, and ending with Elchies, sufficiently 

 demonstrates that a slow retrograde movement in position angle is going on, at 

 the rate of about 2° in fifty years, without any sensible change of distance ; a 

 feature at once of true Unary movement. This character too, is further confirmed 

 by the B. A. C. proper motion test ; for according to that, if the stars were opti- 

 cally connected only, the position ought to have decreased 14^", and the distance 

 increased 1", in thirty years, which is absurd. Our new conclusion therefore of 

 binary character for this pair seems confirmed, and is of infinite importance in 

 considering the periodical A^ariations of colour which its members undergo. 



(14.) 70 OPHIUCHI, R.A. 17*" 58°^ 28% D. + 2° 32' 32", January 1, 1862. 



Com- Maerni- ^ , 



, , Y Colour, 



ponents. tude. 



A 



B 



Position. 



Distance. 



Date. 



Authority 



AB 90 6 



a'b 



1779-77 



B}. 



AB 318 48 



2-56 



1804-41 



B}. 



A 



B 



* Struve's note is as follows : — 



" Colorum diversitas ex singulorum dierum consensu indubia est. Probe notandum est has 

 colorum notationes sine ullo prsejudicio esse factas, cum in tot stellis duplicibus observandis occu- 

 patus, nunquam recordarer, quid peculiaris in hoc illave notaverim. Eadern colorum relatio etiam 

 a H^ et S. est reperta, scilicet priorem esse flavescentem, posteriorem non. Eadem denique in 

 observationibus per instrumenta meridiana ssepius annotavi. Insignis est differentia lumiuis in stelH 

 ejusdem proximo splendoris." 



