DETERMINATION OF THE CONSTANT OF ABERRATION.—Comptes rendus for March 16 contains an abstract of a memoir by MM. Lœwy and Puiseux, on determinations of the aberration constant, in which some of the results obtained by M. Lœwy's method are given. Up to 1828, astronomers accepted, as the constant of aberration, values which were comprised between 20″˙255 and 20″˙708, these being respectively due to Delambre and Bessel. Richardson then obtained the value 20″˙446 from a discussion of 4000 observations made with the Greenwich mural circle by his predecessors. In 1843, W. Struve proposed a value almost identical with this, viz. 20″445, as the result of a discussion of his careful observations made in the prime vertical. He estimated the probable error as 0″˙011, and remarked that he did not think any astronomical element had been determined with so great an accuracy. Struve's work was received with much favour, and appeared to render unnecessary, for a number of years, all researches on the same subject. However, in 1844, Baily deduced 20″˙419 as the most probable value, and in after years, Peters, Lundhal, and Lindhagen subjected to a minute discussion all the meridian observations, made at Dorpat and Pulkova, of circumpolar stars. From their researches, a value a little greater than that of Struve was found. Still, when these results were taken in conjunction with the determinations the most worthy of confidence the values 20″˙45 and 20″˙46 were obtained, thus supporting Struve's work. Nyren, from a discussion of observations made by Struve in the prime vertical as material for the study of nutation, derived the value 20″˙43. In 1853, Struve himself proposed to increase his number to 20″˙463 with a probable error of 0″˙017, but the reasons given to justify the change do not appear to be sufficient. Gylden, Wagner, and Nyren's ulterior observations at Pulkova of circumpolar stars gave the higher value 20″˙49. Later, in 1879–82, Nyren made another determination by Struve's method, and used a large number of stars for the investigation. He then found 20″˙540 or 20″˙5l7, according to the method of grouping adopted. More recently, in 1885, Küstner, of Berlin Observatory, found 20″˙313 by Horrebow and Talcott's method. Between these two last numbers, both of which represent a large amount of work executed with much care, the difference is somewhat greater than 0″˙2—that is, about twenty times the probable error estimated by Struve in 1843. This seems to indicate that the astronomers have taken a step backwards. MM. Lœwy and Puiseux do not enumerate the work done on the same subject at Greenwich, the Cape, Washington, and other Observatories, but point out that similar sources of error exist in all the methods employed. Lœwy's method, as is now well known, consists in placing before the object-glass of an equatorial a double plane mirror formed by silvering the sides of a prism of glass. This acts as a sort of compass of strictly constant opening, and brings to the eyes rays which make a constant angle with each other. Pairs of stars separated by a wide angle on the celestial sphere, but which together appear in the field of view, can easily be found, and the variations in relative position due to refraction or aberration can be measured micrometrically with great precision. The adoption of this method leads the authors to the following tentative conclusions:—(1) The number 20″˙445, proposed by Struve, is very near to the truth. It would be premature, in our opinion, to wish to modify it.