92 



KNOWLEDGE 



[May 1, 1891. 



cluster, without auy marked condensation towards a 

 centre, and it would not be opposed to probabilities or un- 

 reasonable to suppose that we might be looking at two 

 clusters superposed or projected on one another in the 

 line of sight ; if this was the case, the smaller stars might 

 belong to one cluster, and the larger to the other and 

 nearer cluster, and we should then not 

 be warranted in assuming that diffe- 

 rences in apparent magnitude amongst 

 the stars of the group must correspond 

 to differences in actual brightness or 

 size of the stars. If there were two 

 such clusters entirely unconnected with 

 one another, we might expect to find a 

 difference in the proper motion or drift 

 of the two families of stars, or a diffe- 

 rence in their spectra, which might 

 enable us to sort out and distinguish 

 the stars belonging to the two groups. 

 The measurement of the relative posi- 

 tions of the stars in the Pleiades 

 group has occupied the attention of 

 several distinguished astronomers. This 

 was one of the first problems which 

 Bessel attacked with the Konigsberg 

 Heliometer. It occupied his attention 

 during about twelve years, until the 

 work was finally completed in 1841. 

 Dr. Wolf of the Paris Observatory, 

 Prof. Simon Newcomb, Prof. Pritchard, 

 and ultimately Dr. Elkin of Yale, have 

 all devoted much time and care to 

 determining the positions of the stars 

 of this group. The accompanying 

 chart shows the proper motions of the 

 chief stars of the group as determined 

 by Dr. Elkin, by comparing his own 

 measures made with a six-inch helio- 

 meter with those made by Bessel nearly half a century 

 previously. The chart is copied from Dr. Elkin's memoir, 

 which forms the first volume of the Transactions of the 

 Astivnomiral Ohsfrcator;/ of Yah' University* It will be 

 seen that there are seven stars with large proper motions 

 towards tlie north-west, all of them, with the exception of 

 Alfijone, being small stars. They are shown with arrows 

 ha%-ing dotted or broken lines on the chart, corresponding 

 to the amoimt and direction of the proper motions. 

 With the exception of Alcyone, the other six lucid stars 

 of the group have comparatively small proper motions. 



In Dr. Elkin's map the larger stars are distinguished 

 bv letters. Both the letters and numbers are those used 

 by Bessel. The map corresponds to the inverted image of 

 the stars as they are seen in the telescope, and it must 

 therefore be turned round through 180° to make it cor- 

 respond with Mr. Isaac Eoberts's photograph, or with the 

 plate given in the January number of KNO^\T.EDGE for 

 1889. It is remarkable that the general direction of drift 



of these seven stars is very similar to the reversed abso- 

 lute motion of Alcyone in 45 years as given by Prof. 

 Newcomb, viz. 2"-61. There seems to be little doubt that 

 these seven stars are only optically associated with the 

 cluster, and have no physical connection with it. Of the 

 remaining 33 stars whose positions were compared, two. 



* It should be mentioned that Professor Pritchard has given a 

 chart of the proper naotions of the stars of the Pleiades group in the 

 Memoirs of the Royal Astronomical Society, vol. slviii. , p. 272, 

 which does not at all agree with that given by Dr. Elkin. But the 

 practical agreement of Dr. Wolf's measures with those of Dr. Elkin, 

 as well as other considerations, seems to show that, although a very 

 small probable error is claimed by Professor Pritchard, Dr. Elkin's 

 map is probably the most reliable. I do not place much rehance on the 

 smaller proper'motions deduced by Dr. Elkin, that do not amount to 

 half a second in a century, for such an error would about correspond 

 to the quarter of a second of probable error of the Konigsberg 

 observations as deduced by the accurate Bessel. 



\'iz. No5- 8 and 25, have a considerable and similar proper 

 motion towards the south-east, which would seem to 

 render it probable that they are associated. The star N"- 8 

 has, according to Prof. Pickering (see 2Ieiiioirs of the 

 American Academy, vol. xi., p. 214), a spectrum which 

 shows the K line broad and differs materially from the first 

 type of spectra exhibited by most of the other members of 

 the group. The spectrum of N"- 25 is uncertain, as it is 

 ovei-lapped by the spectrum of another star ; 27 and 39 

 also have pecuhar spectra. Setting these aside as pro- 

 bably only optically connected with the group, the re- 

 maining stars have only comparatively small proper 

 motions, and seem to have similar spectra of the first type. 

 There appears to be a tendency to community of diift in 

 adjacent parts of the group, such as might weU be ex- 

 hibited by the stars of an irregular cluster moving under 

 mutual attractions. 



Assuming all the other stars, with the exception of the 

 eleven referred to above, to belong to one cluster, they 

 would all be at about the same chstance from the earth, 

 and their actual brightness may be taken to correspond to 

 their apparent brightness. Prof. Pickering has measm-ed 

 the light of 298 of these stars, varying from a little below 

 the third magnitude down to the 15-3 magnitude of the 

 photometric scale.''- There are many fainter stars in the 

 group, though the instrument used by Prof. Pickering 

 did not enable him to measure then- brightness. Thus 

 the Brothers Henry photographed within the same area,. 



* See vol. .-sviii. of the Harvard Annals, p. 202. 



