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General Meeting-April 18-20, 1912

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The General Meeting of 1912 will be held on April 18th to zoth, beginning at 2 p. m. on Thursday, April 18th.

Members desiring to present papers are requested to send to the Secretaries, at as early a date as practicable, and not later than April 10, 1912, the titles of these papers, so that they may be announced in the final programme which will be issued immediately thereafter, and which will give in detail the arrangements for the meeting.

Papers in any department of science come within the scope of the Society, which, as its name indicates, embraces the whole field of useful knowledge.

The Publication Committee, under the rules of the Society, will arrange for the immediate publication of the papers pre. sented.

I. MINIS HAYS

ARTHUR W. GOODSPEED

AMOS P. BROWN

HARRY F. KELLER

Secretaries.

Members who have not as yet sent their photographs to the Society will confer a favor by so doing; cabinet size preferred.

It is requested that all correspondence be addressed

TO THE SECRETARIES OF THE
AMERICAN PHILOSOPHICAL SOCIETY

104 South FIFTH STREET

PHILADELPHIA, U. S. A.

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The problem of determining the depth of the Milky Way, as accurately as possible, is one which has now engaged my attention for over twenty years, and I will therefore take this occasion to bring together the results at which I have arrived, partly because they are of high general interest, and partly because the progress thus made will prove instructive as to the methods which must be adopted for the measurement of the distances of the most remote objects of the sidereal universe. Here we have to deal with distances so immense that the method of annual parallaxes, commonly used for the stars comparatively near the sun, utterly fails; and recourse must be had to other methods which will serve for the greatest distances to which our modern giant telescopes can penetrate.

Alpha Centauri, the nearest of the fixed stars, was also the first to be successfully measured for parallax, by Thomas Henderson, of the Cape of Good Hope, in 1831; but the work was not reduced till January, 1839, and meanwhile Bessel had measured the parallax of 61 Cygni in 1838 and promptly published the result of his triumph.

PROC, AMER, PHIL. SOC., LI, 203 A, PRINTED MARCH 16, 1912.

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Of late years astronomers have given greatly increased attention to the question of the distances of the stars, and systematic campaigns of the most laborious kind have been carried on by Gill; Elkin and Chase, of Yale; Kapteyn, of Groningen; and Schlesinger, at the Yerkes Observatory, Chicago. Some 350 stars have now been studied by the standard method of parallaxes, and for most of these objects, perhaps about 200 in number, fairly satisfactory data have been deduced; but the method can be extended only to stars within less than 100 light-years of our sun, and is therefore very limited in its applicability, owing to the small diameter of the earth's orbit, and the insensible effects of the annual displacements resulting from the orbital motion of our planet. As nature herself has fixed the limits of this method, astronomers have naturally cast about for other methods of greater generality and have finally developed processes of surprising power, of which an account will be given in the present paper.

§ 1. OUTLINE OF THE METHODS ADOPTED. Among previous investigators who have occupied themselves with the difficult problem of the profundity of the Milky Way the first place will be universally assigned to the incomparable Sir William Herschel, who extended his researches over many years, and reached results which were for a time accepted, but have been rejected for three quarters of a century, and yet are now proved to be essentially correct. It is very remarkable and exceedingly unfortunate that Herschel's conclusions have been generally rejected by his son, Sir John Herschel, and other astronomers during the past seventy-five years. But before discussing the circumstances which led to this outcome I shall recall the modern attempts at the solution of the problem of determining distances in the Milky Way.

After the spectroscope came into use and Huggins had applied Doeppler's principle to the motion in the line of sight (1868) it was pointed out by Fox Talbot in 1871 (Brit. Assoc. Report, 1871, P. 34, Pt. II.) that the possibility existed of determining the absolute dimensions of the orbit of a pair of binary stars which had a known

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