318 ADDRESS ON PRESENTING THE GOLD MEDAL OF THE [42 



The constant of aberration obtained by taking the mean of the several 

 results for the different stars is 20" '481, which differs only 0"'036 from 

 the definitive value found by M. Struve. The smallness of this difference 

 gives great confidence as to the accuracy of the results for parallax, as 

 there is no reason why the aberration should be found more accurately 

 than the parallax. 



Another strong confirmation is afforded by the fact, that the parallax 

 of 61 Cygni determined by M. Peters is absolutely identical with that 

 found by Bessel by means of the heliometer. 



The last part of M. Peters' paper treats of the mean value of the 

 parallax of stars of the second magnitude. M. Peters finds that there are 

 thirty-five stars whose parallaxes are determined with sufficient accuracy to 

 serve as a basis in this research. Of these, however, he excludes two stars 

 which have very large proper motions, Gl Cyyni and 1830 Groombridge, as 

 exceptional, and therefore not properly to be included when an average is 

 the quantity sought. Struve's scale of relative distances of stars of different 

 magnitudes is employed in combining the observed parallaxes for different 

 stars, although the final i^esult is nearly independent of the assumed scale, 

 inasmuch as the second magnitude is nearly the mean of all the magnitudes 

 of the stars employed. 



M. Peters shews his usual skill in estimating the probable errors which 

 may arise from the defects of the hypotheses employed, such as that of 

 the same absolute brightness of the stars, as well as from the errors of 

 the observed parallaxes ; and he finally arrives at the result, that the most 

 probable value of the mean parallax of stars of the second magnitude is 

 0"'11G, and that the probable error of this determination is only 0"'014. 



M. Peters closes his paper with a most interesting result, deduced by 

 combining his own researches with those of M. Otto Struve respecting the 

 solar motion. M. Otto Struve finds that the annual apparent motion of 

 the Sun, as seen at right angles from a point at the mean distance of 

 stars of the first magnitude, is 0"'339. Now, according to M. Peters, the 

 mean parallax of a star of the first magnitude is 0"'209 ; so that we are 

 able to turn the former result into absolute measure. Thus the annual 

 motion of the Sun with respect to the great body of the surrounding- 

 stars is equal to T623 times the radius of the Earth's orbit. 



