/I? 
PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 1 APRIL 15. 1915 Number 4 
PHOTOGRAPHIC DETERMINATION OF STELLAR PARAL- 
LAXES WITH THE 60-INCH REFLECTOR 
By Adriaan van Maanen 
MOUNT WILSON SOLAR OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON 
Presented to the Academy, March 4, 1915 
The determination of stellar parallaxes had been attempted from the 
days of Tycho Brahe down, but without success until Bessel in 1838 
succeeded in demonstrating and measuring the parallax of 61 Cygni. 
Since that time the work has been advancing steadily, first with the use of 
the meridian-circle, heliometer, and micrometer, until in 1886 Pritchard 
applied photography with great success. Although several astrono- 
mers have given much time to the problem, results are still compara- 
tively few. In 1910 Kapteyn and Weersma published a list of well- 
determined parallaxes, which, although nearly complete, contains only 
365 stars. The accuracy of these parallaxes varies considerably; their 
probable errors are anywhere between 0".004 and 0'M51, while the 
mean probable error is 0".032. Since then a few lists of parallax determi- 
nations have been published, which show a good improvement; the Hst 
of Slocum and Mitchell (14 stars) has a mean probable error of 0".011; 
that of Miller (8 stars) of 0^.01 1. 
The material pubHshed so far cannot, however, help us very much in 
forming an idea of the distribution of the stars in space, as it is very 
one-sided. Most of the stars were chosen on account of their brightness 
or their large proper motions; but for the distribution of stars in space, 
it is clear, that we need as well the parallaxes of stars which are not 
supposed to be our nearest neighbors. But here arises a new difficulty; 
the quantities sought become so small, that only the greatest accuracy 
in their determination can give useful results. According to Kapteyn 
the mean parallax of a star of, for instance, the sixth magnitude and a 
187 
