178 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1916. 



not after the method of a century ago. The observations were 

 strictly differential, the general plan being to selett two faint com- 

 parison stars, one immediately preceding and the other immediately 

 following the parallax star, and to determine the difference in right 

 ascension, the observation of the three stars occupying about five 

 minutes. Here, as in the case of the Yale heliometer work, a large 

 proportion of the resulting parallaxes are negative ; somewhat more 

 than half, however, were found to have a measurable parallax. The 

 average 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 Table III, which con- 

 tains also a brief statement of the discoveries made during the 

 jjreceding century, due chiefly to efforts to measure stellar parallaxes. 



Table III. — Apin-oximate number of known stellar parallaxes. 



1 No parallax. 



A generation ago photography entered the field of stellar parallax 

 work, and has outdistanced all the previously employed methods for 

 efficiency. In 1911 two publications appeared giving the results of 

 photographic stellar parallax work, one b}^ Russell, giving the paral- 

 laxes of 1:0 stars from photographs taken by Hinks and himself at 

 Cambridge, England, the other by Schlesinger, giving the parallaxes 

 of 25 stars from photographs taken mostly by himself at the Yerkes 

 Observatory, Williams Bay, Wis. In speaking of these two series 

 of observations. Sir David Gill said: 



On tlie whole, the Cambridge results, wheu a sufficient number of plates have 

 been taken and when the comparison stars are symmetrically arranged, 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 char- 

 acter of the Yerkes instrument "the number of stellar parallaxes 

 that can be determined 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." 



