482 



SCIENCE 



[N. S. Vol. XLIII. No. 1110 



somewhat more than half of the 245 stars 

 have a measurable parallax. 



Another series of stellar parallax obser- 

 vations, comparable in extent with the one 

 just mentioned, is that of Flint at the 

 Washburn Observatory. This series in- 

 cludes 203 stars and extended from 1893 to 

 1905. These observations were made with 

 a meridian circle, but not after the method 

 of a century ago. The observations were 

 strictly differential, the general plan being 

 to select two faint comparison stars, one 

 immediately preceding and the other imme- 

 diately following the parallax star, and to 

 determine the difference in right ascension, 

 the observation of the three stars occupying 

 about 5 minutes. Here as in the case of the 

 Tale heliometer work a large proportion of 

 the resulting parallaxes are negative ; some- 

 what more than half, however, were found 

 to have a measurable parallax. The aver- 

 age probable error of a parallax was the 

 same in each of these two pieces of work, 

 about 0".03. The progress of the work 

 during the last two or three generations is 

 given in the following table, which contains 

 also a brief statement of the discoveries 

 made during the preceding century, due 

 chiefly to efforts to measure stellar par- 

 allaxes. 



APPROXIMATE NUMBER OF KNOWN STELLAR 

 PARALLAXES 



A generation ago photography entered 

 the field of stellar parallax work, and has 

 outdistanced all the previously employed 

 methods for efficiency. In 1911, two pub- 



lications appeared giving the results of 

 photographic stellar parallax work, one by 

 Russell, giving the parallaxes of forty stars 

 from photographs taken by Hinks and him- 

 self at Cambridge, England, the other by 

 Schlesinger, giving the parallaxes of 

 twenty-five stars from photographs taken 

 mostly by himself at the Yerkes Observa- 

 tory, Williams Bay, Wisconsin. In speak- 

 ing of these two series of observations, Sir 

 David Gill said. 



On the wholej the Cambridge results, when a 

 sufi&oient number of plates have been taken, and 

 when the comparison stars are symmetrically ar- 

 ranged, give results of an accuracy which, but for 

 the wonderful precision of the Yerkes observations, 

 would have been regarded as of the highest class. 



Schlesinger has shown that with a telescope 

 of the size and character of the Yerkes in- 

 strument 



the number of stellar parallaxes that can be de- 

 termined per annum, with an average probable 

 error of 0."013, will in the long run be about equal 

 to the number of clear nights available for the 

 work. 



In other words, the Yerkes 40-inch equa- 

 torial used photographically determines 

 stellar parallaxes with one tenth the labor 

 required with a heliometer and with twice 

 the accuracy. 



In July, 1913, stellar parallax work was 

 undertaken with the 60-inch reflector of the 

 Mount Wilson Solar Observatory, and at 

 the meeting of the American Astronomical 

 Society at San Francisco in August, 1915, 

 a report on that work was made. The par- 

 allaxes of thirteen stars had been deter- 

 mined, with a maximum probable error of 

 0".010 and an average probable error of less 

 than 0".006, giving twice the accuracy of 

 the Schlesinger results with the Yerkes 40- 

 inch and from three to five times that ob- 

 tained fifteen years ago. What may we not 

 expect when the 100-inch reflector gets to 

 work on Mt. Wilson. 



