O PRESIDENT S ADDRESS. 



most recent determinations made with the great telescopes of 

 America, and in particular the 40-inch refractor of the Yerkes 

 Observatory, have a probable error of about o".oi, or ten times less 

 than the usual visual method, and Dr. Van Maanen, using photo- 

 giaphs taken with the 60-inch reflector of the Mount Wilson 

 Observatory, has reduced this probable error to o".oo6, or about 

 a hundred and seventieth part of a second of arc. As regards 

 the measurements of small quantities this is a wonderful achieve- 

 ment, but delicate as these measurements are, they are too coarse 

 to tell us much about the distances of the stars. 



Let us consider several recent sets of parallax measures : — 



1. Van Maanen's list of five stars is as follows: — 



Star. Parallax. Probable Error. 



96 0.026 +0.007 



672 — 0.009 0.004 



1549 O.OOI 0.002 



2921 O.07S 0.006 



2^2T,2, 0.003 o.oio 



2. In two recent lists we find parallaxes for 61 Cygnus, the 

 star for which Bessel first found a parallax. 



Probable 

 Authority. Paralla.v. Error. 



Miller, 24-in telescope. Sproul Observatory . . . o".30i ±o''.oio 



Slocum & Mitchell, 40-in. telescope, Yerkes Obs. 0^.272 o".oo5 



The negative parallax in Van Maanen's list would mean that 

 the star was actually more distant than its comparison stars, 

 which is at least unlikely, and in two other cases it will be seen 

 that the parallaxes found are smaller than their probable errors. 

 Somewhat similarly, in the case of 61 Cygnus, although the two 

 parallaxes found agree very well, they differ by much more than 

 their probable errors. 



3. In the recent most considerable list of stellar parallaxes 

 published (Slocum & Mitchell, Popular Astronomy, 1914 March), 

 out of twenty-eight results, eight are negative parallaxes* and 

 another four are smaller than their probable errors ; yet the list 

 is one of stars selected for large proper motion or some other 

 peculiarity which indicated a measurable parallax. 



These three sets show us that, valuable as the photographic 

 method is, it is to be feared that it will also soon work out its 

 rich lodes. So it does not take us much further. In this way 

 the direct attack by parallactic displacement will reveal perhaps 

 some one or two hundred parallaxes ; but we would learn nothing 

 as to the distances of the great mass of stars, except what we 

 already know, namely, that the distances are tremendous. 



*In this list the parallaxes of the two components of South 435 (5 '443) 

 [3h. 40m. + 41°] are measured. These two stars form a physical system 

 as they are travelling together through space witli an annual proper motion 

 of i".4 in the direction of 149°. The parallaxes found are — 



ist star — o".oi6 +0.0TI 



2nd star ... ... -ho".oio +0.008 



which are contradictory, because they signifiy that the first star was more 

 distant, and the second star nearer, than the comparison stars. It is 

 likely enough that even, photographically, our present limit is about o".03, 

 corresponding to a distance of al)out 30 radials. 



