372 AstronoDiical Socieii/. 



honour of the University of Oxford, twenty-three sheets of them are 

 ah'eady printed ; and that the vohime will be presented to the public 

 with as little delay as possible, under the superintendence of Professoi* 

 Rigaud. 



Till within these few years, the constant, as determined by Bradley, 

 was universally employed in all our astronomical reductions j re- 

 cently, however, astronomers have re-investigated it. Delambre, 

 from the eclipses of Jupiter's satellites, regards it as 20 ','25. Bessel, 

 from Bradley's observations made at the Royal Observatory, after he 

 was appointed Astronomer Royal, has deduced for the constant 20",68. 

 Lindenau, by comparisons of Bradley's, Maskclyne's, Bessel's, and 

 Pond's observations of Polaris, has obtained for it 20 ",61. Brinkley, 

 from his own observations, considers it as 20",37. Whilst Struve, by 

 his observations, draws almost the same inference, namely 20",35. 



Such were the results most entitled to our confidence, when Mr. 

 Richardson, in the hours allotted to him for repose or recreation, un- 

 dertook those labours which form the subject of" our present considera- 

 tion, and of which the following is a brief outline. A second mural circle 

 by Jones, after the model of that of Troughton's, having been erected 

 at the Royal Observatory, in the April of 1825, corresponding obser- 

 vations with the two instruments were carried on simultaneously ; 

 they were confined, indeed, to a few stars, but every precaution to 

 render them as accurate as possible was adopted. Thus, the index 

 error of each instrument was ascertained by observing the same star 

 alternately, by direct vision and reflection ; each pair, therefore, giving 

 to its resi)ective instrument one horizontal point perfectly indepen- 

 dent of astronomical tables, the final accuracy of the determination 

 of the index error being directly as the number of pairs observed. 

 Throughout the observations, the place of each star was arrived at by 

 reference to the six microscopes of each circle ; care also being taken 

 to equalize, as nearly as possible, the temperature of the observatory 

 with that of the external air, so that errors to any extent, arising from 

 partial expansions or erroneous divisions of the instrument, were 

 efiectually excluded. 



From such unexceptionable data, fourteen stars were selected by 

 Mr. Richardson as the fittest for his purpose, being those the least 

 affected by refraction, and the most affected by abenation, so that 

 the errors of observation might have the minimum influence upon the 

 results. Upwards of 4000 observations he separately discussed, and 

 in no instance was the actual aberration of each observed star less 

 than 14"; and the conclusion to which he has arrived is, from Trough- 

 ton's circle, tiiat the Constant of Aberration is 20", 505 ; whilst by 

 Jones's it is -0"',502, the one differing from the other but three-thou- 

 sandths of a second. 



Hook, ill searching for parallax, was misled by his instrument, 

 Bradley, in detecting it, was unsuccessful, but discovered aberration. 

 Hook's instrument was the work of his own hands ; Bradley's was 

 the work of Graham. Sensible that much of his astronomical glory 

 was referable to the accuracy of his instrument, the amateur astrona- 

 iTier of ^\'anstead was ever ready lo acknowledge it ; and when we 



consider 



