42] ROYAL ASTRONOMICAL SOCIETY TO M. PETERS. 315 



the new value of nutation. The rate of the diminution so found agrees 

 very well with that found by M. Le Verrier from theory, the difference 

 not amounting to one second in a century. The true value of the obliquity 

 of the ecliptic at a given epoch cannot, however, be considered as definitively 

 settled, in consequence of the puzzling constant differences between the 

 declinations determined at different observatories. For instance, the obliquity 

 given by the mean of several years' observations at Greenwich exceeds by 

 rather more than one second the obliquity for the same epoch given by 

 M. Peters' investigations. 



M. Peters' researches respecting precession are based on the results of 

 M. Otto Struve's paper, which obtained our medal on a former occasion, 

 combined with M. Le Verrier's detei'mination of the secular change in the 

 position of the ecliptic. 



M. Otto Struve determines, independently, by observation, the values 

 of two constants on which the precessions in right ascension and declination 

 depend. Now, theory establishes a relation between these constants, and 

 M. Peters is thereby enabled to find the most probable values which result 

 from the combination of the observed values, and thence to derive complete 

 formulae for precession applicable to any given epoch. 



I have no hesitation in regarding M. Peters' results, with respect both 

 to precession and nutation, as definitive for the present state of astronomy. 



I now come to M. Peters' second paper, which relates to the delicate 

 subject of the parallax of the fixed stars. 



The first part of this important paper contains an historical and critical 

 review of the researches of astronomers respecting parallax from the time 

 of Tycho to the year 1842. The second treats of the parallaxes of several 

 stars as determined by M. Peters' own observations, made at Pulkowa by 

 means of the great vertical circle of Ertel. In the third part, the results 

 of the two former are applied to determine the mean parallax of stars of 

 the second magnitude. 



The historical part is drawn up with great care, and contains many 

 curious and interesting discussions on particular points. For instance, 

 M. Peters shews that the coefficient of aberration may be obtained with 

 great accuracy from Flamsteed's observations of the zenith distance of the 

 pole-star. The probable error of a single observation is found to be only 

 6", which gives a far higher idea of the accuracy of Flamsteed's observations 



402 



