September 1, 1899.] 



KNOWLEDGE. 



201 



From a consideration of the various recorded magnitudes, 

 C. H. F. Peters came to the conclusion that the star is 

 probably variable with a long period. The fact of its 

 having been seen at all by Sufi proves beyond reasonable 

 doubt that it must have been brighter then than it is at 

 present. I estimated it about 7th magnitude in August, 

 1877, and November, 1882. 



r, Crateris. — This star was rated 4-3 by Ptolemy, 5-6 by 

 Sufi, 4 by Ulugh Beigh, Tycho Brahe, and Hevelius, 6 by 

 Argelander and Heis, 5-4 by Gould, and only 6-7 by 

 Houzeau. It was measured 5'01 at Harvard. The star 

 seems certainly variable to a considerable extent, but the 

 period may be very long ; otherwise, it seems impossible 

 to explain how a star which was seen only 6;^ magnitude 

 by Houzeau — or barely visible to the naked eye — should 

 have been rated fourth magnitude by Tycho Brahe, who 

 was a very accurate observer. 



" The Story of 9 Eridani " has been already told in a 

 most interesting paper in Knowledge (July, 1893) by 

 Dr. Anderson, the discoverer of Nova Aurigje. I fully 

 agree with Dr. Anderson that 5 Eridani is certainly 

 identical with the " Last in the River " of Ptolemy, and 

 the Achernar of Sufi, and that consequently the star has 

 undoubtedly faded from the first magnitude to about the 

 third since the tenth century. It is one of Sufi's thirteen 

 first magnitude stars, and his clear description of its 

 position places its identity beyond all doubt. In recent 

 years the star has been suspected of variation in its light. 

 It is a splendid double star — one of the finest in the 

 heavens. 



We will now consider some telescopic objects. The 

 famous variable star Algol has several faint companions. 

 One of these was discovered in 1787 by Schroter, who 

 strongly suspected it to be variable. Sadler thought it 

 variable from 10th to 14th magnitude in some short period. 

 A writer in Nature (February -iOth, 1879) stated that he 

 failed to see any trace of the star on several fine nights in 

 the early part of 1874, using a seven-inch refractor, but 

 on September 9th of the same year he saw the companion 

 very distinctly with the same instrument. It was measured 

 by Talmage at the Leyton Observatory on October 2nd, 

 1874, and estimated 11-12 ; and in 1878 by Burnham, 

 who found three other fainter companions, one not far 

 from Schroter's companion and forming with it a wide 

 double star. Franks found it " easy enough " with an 

 eleven and a-quarter inch reflector on January 11th, 1885, 

 and about two magnitudes brighter than Burnham's faint 

 companion. If variability is confirmed it will be very 

 interesting, as a variable companion to a known variable 

 star would be a rather unique object. There is, however, 

 a somewhat similar case in the star S (15) Monocerotis, 

 which is supposed to be a short period variable. A distant 

 companion to this star was rated 8^ magnitude by 

 Main in 1863, but only 12 magnitude by Sadler in 1875. 

 Observations by Mr. Tarrant in March and April, 1888, 

 showed a variation of \\ magnitude in about three 

 weeks. On March 28th, 1889, observing with a three- 

 inch refractor, I found the suspected variable about one 

 magnitude brighter than another companion a little to the 

 west of it. 



l UrsiB Majoris.^In Cmnptes Betidus, Vol. XIII., p. 438, 

 Mildler states that he foimd the companion to this bright 

 star invisible on April 18th, 1841. About an hour after- 

 wards it was again visible, and he suggests that the 

 companion is variable " comme Algol, mais probablement 

 avec une periode beaucoup plus longue." His account 

 leaves it uncertain whether Alcor or the closer companion 

 is referred to, but as he seems to have been using a tele- 

 scope at the time, probably the close companion is meant. 



The naked eye companion Alcor has also been suspected 

 of variation in brightness. 



7 Virginis. — The components of this famous binary 

 star were considered by Struve to be alternately variable 

 in brightness. His observations in the years 1851 and 

 1852 show that sometimes the component stars were 

 exactly equal in brilliancy, and that sometimes the 

 Southern star — the one generally taken as the primary 

 star —was from 0-2 to 0-7 magnitude brighter than the 

 other. There seems to be little doubt that some variation 

 really takes place in the relative brightness of the pair. 

 This is clearly shown by the measures of position angle. 

 For example, in the year 1886, Prof. Hall recorded the 

 position angle as 154-9', evidently measuring from the 

 Northern Star as the brighter of the pair ; while in 1887, 

 Schiaparelli gives 334'2°, indicating that he considered the 

 Southern Star as the primary or brighter of the two. 

 Burnham found 153-4° in 1889, and Dr. See 332-50° in 

 1891. The period of variation would seem to be short, 

 for 0. Struve found the Southern Star half a magnitude 

 brighter than the other on April 3rd, 1852, while on April 

 2ilth of the same year he found them " perfectly equal." 

 He thought the variation was about 07 of a magnitude, 

 but that the climate of Poulkova, where he observed, was 

 not good enough for such observations. The variation is 

 very interesting, and the question should be thoroughly 

 investigated with a good telescope, 



Pollux (/? (leminorum). — Admiral Smyth measured two 

 faint and distant companions to this bright star, and says 

 (Bedford Catalogue, p. 187): " A, 2, orange tinge ; B, 12i, 

 ash-coloured; C, 11 j, pale violet; and it has a minute 

 comes to the .•>■. p., which, though unnoticed in former 

 registers, is certainly now (1832) as bright as C ; these 

 companions form a neat triangle." And he adds in a foot- 

 note : " While this is in the press, the Rev. W. R. Dawes 

 has shown me an exact diagram which he made of the 

 object, January 1st, 1829, with a three and a-half foot 

 achromatic, charged with a Huygenian eyepiece magnifying 

 two hundred times. With this instrument he saw the 

 three companions very distinctly, although two only were 

 visible, and that but on remarkably fine nights, in Sir 

 James South's seven foot equatorial, with an aperture of 

 five inches." In the Monthly Notices, E. A. S., for April, 

 1861, the Rev. T. W. Webb says that with his five and a-half 

 inch object glass the third star appears " as much inferior 

 to B, 12m., as B is below C ; and as Sir James South's 

 equatorial of five inches had shown but two companions 

 some years before the date of the Bedford Catalogue, 

 there is, perhaps, grounds to suspect a variation in its 

 light." Burnham, in 1879, rated Smyth's star C as 

 9th magnitude, and the third star 9|. In 1878 he 

 called B 10th magnitude. On January 30, 1880, observ- 

 ing Pollux with a three-inch refractor and power one 

 hundred and thirty-three, I found C quite plain and B 

 tolerably so, with Pollux in the field of view ; D, Smyth's 

 third star, only seen with Pollux just out of the field ; C 

 about one magnitude brighter than B, and B one a-half 

 or two magnitudes brighter than D, which was very 

 faint. To my eye C seemed about 10th magnitude 

 (Smyth's scale), B 11th magnitude, and D about 12^. 

 On March 31st, 1881, I again examined the star with 

 the same instrument and power, and found C brighter 

 than B, and D very faint, hardly visible. D was not 

 visible with a power of eighty-three. If D was ever equal 

 to C, as expressly stated by Admiral Smyth, it must 

 certainly be variable. 



There are many other telescopic objects which have 

 been suspected of variable light, and some of these I may 

 deal with in a future paper. 



