236 REPORT ON THE TRANSACTIONS OF THE SOCIETY OF 



the duration of a revolution, and the determination of which is de- 

 ducible to quite a close approximation from this single appearance 

 by a comparison of all the observed places after and before the pas- 

 sage to the perihelion. M. Plantamour has entered into many details 

 on the physical appearance of the comet, which he has reproduced in 

 a series of drawings. He has described the presence of an obscure 

 space situated immediately behind the nucleus in the part opposed 

 to the sun; this obscure space, which often appeared darker than the 

 ground of the sky, varied sensibly in its form and extent during the 

 course of the observations; and he remarks that it is equally to be 

 observed in the delineations of Halley's comet published by Bessel. 

 Lastly, he has added some remarks on the size, both apparent and real, 

 on the form and direction of the tail, the apparent length of which 

 was 41° on the 5th October, and the linear length 13.5 millions of 

 leagues, (25 to the degree,) while on the 13th October the apparent 

 length was not more than 32°, and the linear length 10.5 millions of 

 leagues. M. Thury and M. Wartman have also described some pecu- 

 liarities relative to the light emitted by this body, and M. Wartman, 

 the younger, as likewise M. De la Rive, have pointed out the analogy 

 which the bifurcation of the tail of the comet into two parts, separated 

 by an obscure space, presents to the appearance which flames affect 

 under the action of the magnet — an analogy which might, perhaps, 

 confirm the idea already promulgated, and more particularly by 

 Bessel, of a magnetic influence of the sun. • 



An astronomical labor of an entirely different kind is that which 

 M. Ritter has been engaged in, being the calculation of observations 

 of the fixed stars. This work has been undertaken with the view of 

 ascertaining the cause of the abnormal result presented by the re- 

 duction of observations of the star y of the Dragon, as made by M. 

 Main in the XXIV volume of the Memoirs of the Astronomical Society 

 of London. After different calculations, made with great care and 

 checked by numerous verifications, M. Ritter continues to find, like 

 M. Main, a negative parallax, but of less value, though he took 

 account, which M. Main did not, of the influence of the ellipticity of 

 the earth's orbit in the phenomena of the aberration and the parallax. 

 It results therefrom that the observations are infected with errors, 

 proceeding, no doubt, from a defect in the stability of the instrument. 

 In effect, as M. Plantamour has remarked, there are reasons for doubt- 

 ing the exact steadiness of the old zenith sector of Greenwich, and in 

 submitting the observations to calculation it were to be wished that 

 those only were employed which are made with the new sector of M. 

 Airy. At all events, it has been made to appear by M. Ritter's labors 

 that the calculation of an elliptical parallax essentially modifies the 

 results found by the circular parallax, which demonstrates the abso- 

 lute necessity of taking account of the ellipticity of the earth's orbit 

 in calculations of this kind. 



Independently of the original memoirs just spoken of, the society 

 has had several interesting communications on astronomy. Professor 

 Gautier has kept it constantly advised of the researches made by 

 foreign astronomers, particularly those of M. Carrington on the exist- 



