150 Astronomical Society. 
therefore introduced as an unknown quantity into the equations of 
condition ; and, from the value which is obtained, we may judge of 
the degree of accuracy with which the parallax is determined. 
On resolving, by the method of minimum squares, the two sets 
of equations, and combining the results according to their relative 
weights, the greatest effect of parallax in declination is found, from 
the whole of the 231 observations, = + 0'''15 ; and the greatest 
effect of aberration in declination, = 13"’07. These quantities are 
to the total effect of parallax and aberration in the proportion of 
13"T3 to 20"’50, whence the final results are — 
Parallax of Sirius (or the angle subtended by the radius of 
the earth’s orbit, at a distance equal to that of the star). = 0''’23 
Constant of aberration s= 20*41 
The error of this determination of the parallax may be estimated 
not to exceed a quarter of a second, as it is almost certain that the 
constant of aberration is not in error to a greater amount. On the 
whole, it may be concluded that the parallax of Sirius is not greater 
than half a second of space, and that it is probably much less. 
A Catalogue of Twenty-seven Stars of the Pleiades. By M. Bes- 
sel, Director of the Observatory Konigsberg. 
The catalogue was computed by M. Bessel from meridian obser- 
vations made by himself and his assistant. Dr. Busche. It contains 
the positions, annual precession, and its secular variations in JR. and 
declination, together with the proper motions, and a comparison 
with Piazzi’s catalogue. 
In a letter addressed to Mr. Baily, containing the above catalogue, 
M. Bessel announces, that the observations respecting the parallax 
of 61 Cygni'^ have been continued through a second year; and that 
the result of this new series will agree very nearly vdth that of the 
first. The publication of the observations will be delayed for a few 
months, in order to obtain a more certain determination of the pro- 
per motions which the two small stars compared seem to possess ; 
and he adds, that although the weight of the former result was suffi- 
ciently great to leave no doubt about the real existence of the par- 
allax, it is gratifying to see its quantity so very nearly confirmed by 
a second series of observations. 
A Letter from M. Valz, Director of the Observatory at Mar- 
seilles, to the President, Sir J. F. W. Herschel, Bart., relative to the 
Variation of the Apparent Diameter of Encke’s Comet. 
After adverting to some objections suggested by Sir John Her- 
schel (Memoirs of the Royal Astronomical Society, vol. vi. p. 102f) 
to the theory by which M. Valz explains the changes observed in 
the apparent diameters of some comets, when near their perihelia, 
namely, the condensation of volume produced by the pressure of an 
ethereal medium growing more dense, in the vicinity of the sun, the 
* Abstracts of M. Bessel’s observations on the parallax of the star 6l 
Cygni\y\\\he found in Lond. and Edinb. Phil. Mag. vol. xiv. p. 68. 226. — Ed. 
t An abstract of Sir J. Herschel’s paper appeared in Lond. and Edinb. 
Phil. Mag. vol. ii. p. 222. — Edit. 
