The stars of the Pleiades have, from the earliest times, attracted the special notice of observers, whether savage or civilized.  Hence, on the one hand, their prominence in stellar mythology all over the world; on the other, their unique interest for purposes of scientific study and comparison.  They constitute an undoubted cluster; that is to say, they are really, and not simply in appearance, grouped together in space, so as to fall under the sway of prevailing mutual influences.  And since there is, perhaps, no other stellar cluster so near the sun, the chance of perceptible displacements among them in a moderate lapse of time is greater than in any other similar case.  Authentic data regarding them, besides, have now been so long garnered that their fruit may confidently be expected at least to begin to ripen.

Dr. Elkin determined, accordingly, to repeat the survey of the Pleiades executed by Bessel at Konigsberg during about twelve years previous to 1841.  Wolf and Pritchard had, it is true, been beforehand with him; but the wide scattering of the grouped stars puts the filar micrometer at a disadvantage in measuring them, producing minute errors which the arduous conditions of the problem render of serious account.  The heliometer, there can be no doubt, is the special instrument for the purpose, and it was, moreover, that employed by Bessel; so that the Konigsberg and Yale results are comparable in a stricter sense than any others so far obtained.

One of Bessel’s fifty-three stars was omitted by Dr. Elkin as too faint for accurate determination.  He added, however, seventeen stars from the Bonn Durchmusterung, so that his list comprised sixty-nine, down to 9.2 magnitude.  Two independent triangulations were executed by him in 1884-85.  For the first, four stars situated near the outskirts of the group, and marking the angles of quadrilateral by which it was inclosed, were chosen as reference points.  The second rested upon measures of distance and position angle outward from Alcyone ([eta] Tauri).  Thus, two wholly unconnected sets of positions were secured, the close accordance of which testified strongly to the high quality of the entire work.  They were combined, with nearly equal weights, in the final results.  A fresh reduction of the Konigsberg observations, necessitated by recent improvements in the value of some of the corrections employed, was the preliminary to their comparison with those made, after an interval of forty-five years, at Yale College.  The conclusions thus laboriously arrived at are not devoid of significance, and appear perfectly secure, so far as they go.

It has been known for some time that the stars of the Pleiades possess a small identical proper motion.  Its direction, as ascertained by Newcomb in 1878, is about south-southeast; its amount is somewhat less than six seconds of arc in a century.  The double star 61 Cygni, in fact, is displaced very nearly as much in one year as Alcyone with its train in one hundred.  Nor is there much probability that this slow secular shifting is other than apparent; since it pretty accurately reverses the course of the sun’s translation through space, it may be presumed that the backward current of movement in which the Pleiades seem to float is purely an effect of our own onward traveling.