578 Royal Astronomical Society, 



meter used has four thick wires, two of which are parallel to the 

 screw, and therefore at right angles to the webs, and fixed at an 

 angular distance of 7" between their adjacent edges, and the other 

 two parallel to the webs, and moveable with them. By means of 

 these four thick wires a small square was formed, as nearly as pos- 

 sible in the centre of the field, and the nucleus of the comet placed 

 in it for observation. 



The nucleus resembled a star of about the 10th or 11th magni- 

 tude, abruptly diffusing itself into the nebulosity around it. It was 

 very decided, with powers 63 and 105, the latter of which was ge- 

 nerally employed in the observations. The first day of observation 

 of the comet was Nov. 14, when, as well as on Nov. 16, 19, and 24, 

 the stellar nucleus was remarked as distinctly visible. On Nov. 19 

 it appeared rather eccentric on the north following side. After 

 Dec. 3 a succession of cloudy evenings occurred, and the comet was 

 not seen again till Dec. 22, when the stellar nucleus was again visi- 

 ble, and the apparent diameter of the comet was certainly larger. 

 It must also have been brighter, since the observations of it were 

 considered good, though the sky was so hazy that stars of the 4th 

 magnitude were scarcely visible to the naked eye. 



The last night of observation was Dec. 29, when it still exhibited 

 a decided stellar nucleus, and the nebulosity appeared more ex- 

 tended and dense. No observations were made to determine its 

 diameter, but it was estimated at 1£\ At no time has any appear- 

 ance of a tail been suspected. 



The method of observation adopted by Mr. Dawes was that of 

 transit-comparisons in right ascension, and of micrometrical com- 

 parisons in north polar distance with neighbouring stars whenever 

 it was practicable. Most of the comparison- stars have been iden- 

 tified as contained in the Catalogue of Groombridge ; and the 

 author enters greatly into detail respecting the catalogue-places of 

 all his observed stars, preferring the places given in Pond's Cata- 

 logue of 1112 stars, whenever they were found there. 



A table annexed to the paper, and given in No. 14 of the Monthly 

 Notices, exhibits the observed apparent places of the comet. 



May 14. — The following communications were read : — 



Description of a Dioptric Telescope, and of a Micrometrical Lu- 

 nette. By M. Chevalier. Translated by R. W. Rothman, Esq. 



The dioptric telescope constructed and described by M. Chevalier 

 differs from other telescopes chiefly in the arrangement of the ob- 

 ject-glass. This consists of a large achromatic combination of long 

 focus, and of a small one which reunites the rays transmitted by the 

 primary one. The chief advantage derived from this construction 

 is the great diminution of spherical aberration, and, consequently, 

 the power of increasing the aperture of the telescope with the same 

 focal distance. This advantage is obtained by dividing the excess of 

 curvature between the two achromatic combinations, and by calcu- 

 lating the distance between them, so as best to secure the diminu- 

 tion of the spherical aberration and perfection of the achromatism. 

 The author considers that a great advantage is gained by the process 



