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PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES Volume I APRIL 15. 1915 Number 4 PHOTOGRAPHIC DETERMINATION OF STELLAR PARALLAXES WITH THE 60-INCH REFLECTOR By Adriaan van Maanen MOUNT WILSON SOLAR OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON Preseted 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 in1838 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 astronomers have given much time to the problem, results are still comparatively few. In 1910 Kapteyn and Weersma published a list of welldetermined 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'.151, while the mean probable error is 0".032. Since then a few lists of parallax determinations have been published, which show a good improvement; the list of Slocum and Mitchell (14 stars) has a mean probable error of 0'.011; that of Miller (8 stars) of 0'.011. The material published 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