618 Proceedings of the Royal Irish Academy. 



stars, only 400, but these had. been most carefully obseryed in Dorpat 

 from 1822 to 1838, and the resulting positions could be compared with 

 Bradley's positions for the epoch 1755. The interval between the two 

 epochs was thus considerably longer than in the case of Bessel's in- 

 vestigation, and this is a great advantage, as any error in the adopted 

 equinoxes will thus be divided by a larger quantity and become less 

 detrimental. But it detracts greatly from the value of Struve's result, 

 that he has used such a small number of stars. It is easy to see that, 

 if we use a large number of stars, we have to a great extent a right to 

 suppose them uniformly distributed over the celestial sphere, and we 

 may then safely neglect the influence of the proper motion of the solar 

 system in space, as this influence on the stars in a certain part of the 

 sky will be counteracted by the influence on the region diametrically 

 opposite. But 400 stars are of course not uniformly distributed, and 

 Struve was thus obliged to take the proper motion of the sun into 

 account. This he did in a very beautiful manner ; but as the amount 

 of the apparent motion of a star, occasioned by the motion of the sun 

 in space, depends on the distance of the star, Struve had to adopt his 

 father's expressions for the relative distances of stars of the various 

 classes of magnitude, and it will be conceded by everybody that these 

 expressions are anything but certain. They depend on two hypotheses, 

 neither of which are very well founded, viz., the supposition of a 

 uniform distribution in space of the stars of the galaxial system, and 

 the supposition that the apparent brightness of stars generally depends 

 on their distance from us. Unfortunately the influence of the adopted 

 system of relative distances on the final result is not inconsiderable. 

 Struve's result, which is for 1800, 



50" -2411, 



has by degrees come into general use ; but partly on account of the 

 difference between it and Bessel's result, partly owing to the doubtful 

 suppositions on which it to some extent depends, it must be considered 

 very desii'able to try by other means to deduce a new value for the 

 Constant of Precession. This was done in 1869 by Dr. E'yren of Pul- 

 kova, in a paper in the Swedish language,'^ but the value he found 

 differed in a remarkable manner from the results of Bessel and 

 Struve. 



IS'yren adopted a new plan. Instead of using the brighter stars 

 such as generally occur in Fundamental Catalogues, he chose the Eight 

 Ascensions of the large number of telescopic stars round the equator 

 (between ±15° declination) which could be found both in "Weisse's 

 Catalogue (deduced from observations made by Bessel in 1821-25) 

 and in Schjellerup's Catalogue, founded on observations made by this 

 astronomer in the years 1861-63. As the positions for these 5000 



^ Translated in tlie Bulletin de I'Acad. Imp. de St. Petersbourg, 1870. 



