Astronomical Society. 151 



in question taken with the same micrometer was 42"*34. Call- 

 ing this diameter x, and the latitude on the planet, Z, we have 



A . B 

 x = '■ 7, and the numerical result is x = 42"*38, 



(A 2 sin 8 /+B 2 cos 2 0^ 

 differing only 0" # 04- from the measurement. Most probably it is 

 the slanting position of the axes of the ellipse with regard to 

 the vertical circle which causes this illusion. 



Lastly, there was terminated on the same evening, an " Ac- 

 count of some Observations made with a twenty-feet Reflecting 

 Telescope, by J. F. W. Herschel," Esq. Foreign Secretary 

 of this Society. This valuable communication is divided into 

 four sections. The first contains descriptions and approxi- 

 mate places of 300 new double and triple stars. The telescope 

 with which the observations were made, is one of the " front 

 view" construction; aperture 18 inches, focal length 20 feet. 

 It was constructed in the year 1820, under the joint superin- 

 tendence of Mr. Herschel and his venerable father. Its light 

 with its full aperture enables it to reach the faintest nebulae of the 

 third class, while with an aperture of 10 or 12 inches it serves 

 to define double stars of the first class of an average degree of 

 closeness. Mr. Herschel briefly describes the method of dif- 

 ferences employed in sweejis of the heavens, the modifications 

 introduced into the process on account of Mr. Herschel's being 

 deprived of the valuable assistance of his aunt, Miss Caroline 

 Herschel, his classification and characteristics of the magni- 

 tudes of the stars from the 7th to the 20th inclusive, of which 

 none of the three last can be seen with the least illumination, 

 but comprehend the stars seen or suspected in resolvable ne- 

 bula?. Mr. H. then presents an example of the method in 

 which the business of " a sweep " is conducted, and of the 

 method of obtaining from it the approximate right ascensions 

 and polar distances of the objects which it comprises ; accom- 

 panied by several instructive remarks. The table exhibits, in 

 eight columns, the approximate places of 321 new double 

 and triple stars, for Jan. 1, 1825, with their estimated angles 

 of position, distances, magnitudes, and other particulars. A 

 great many of the double stars tabulated in this paper, exhibit 

 the highly interesting and curious phaenomenon of contrasted 

 colours ; in combinations of white and blue or purple, yellow, 

 orange, or red, large stars, with blue or purple small ones : 

 red and white combinations also sometimes occur, but with 

 less frequency. In all these cases the excess of rays belonging 

 to the less refrangible end of the spectrum falls to the share 

 of the large star, and those of the more refrangible portion to 

 the small. Another fact not less remarkable, and rendering 

 highly probable some other relation than that of mere juxta- 

 position, 



