346 ADDRESS ON PRESENTING THE GOLD MEDAL OF THE [45 



Rendus of December 12, 1853 (tome xxxvii. p. 874), he gives a catalogue 

 of the places of 53 nebulae for the beginning of 1850, selecting such as 

 had well-defined centres or points of greatest brilliancy. It is to be regretted 

 that no details are given respecting either the number of observations on 

 which the places in the catalogue are founded, the mode of observation, or 

 the telescope employed, so that the catalogue itself affords us no means of 

 judging of the degree of accuracy of the places contained in it. 



Professor D'Arrest's first series of observations on the nebulae began in 

 May 1855, and, like M. Laugier's, had for their object the accurate determi- 

 nation of positions for the express purpose of affording means in due time 

 of studying the proper motions of the nebulae, and thence arriving at more 

 certain conclusions respecting the relations between the nebular and sidereal 

 systems than could be attained by the mere contemplation and examination 

 of the objects themselves, even with the aid of the most powerful telescopes. 

 The results of these observations were published in the Transactions of the 

 Royal Saxon Society of Sciences for 1856. The number of nebula? observed 

 amounts to 230. The observations were made at the Leipzig Observatory, 

 of which Professor D'Arrest was then the Director, with the Fraunhofer 

 refractor of 4J French inches in aperture and 6 feet focal length, by means 

 of a Fraunhofer's double ring-micrometer. The magnifying power usually 

 employed was 42 times. The nebulas were thus directly compared with neigh- 

 bouring stars out of Bessel's and Argelander's Zones. In one night usually 

 three and sometimes four transits of a nebula and its comparison-star were 

 observed, the transits being taken alternately in the northern and southern 

 halves of the ring-micrometer. In order to guard against the uncertainty 

 which may still remain in the places of the stars of comparison, Professor 

 D'Arrest often gives, in his description, the observed differences of right 

 ascension and declination. He also often gives the position of the nebula 

 with respect to the nearest stars, frequently those of the 10th and llth 

 magnitude, which must ultimately prove most useful for the determination of 

 the nebula's proper motion. In this last point he followed the excellent 

 practice of Sir John Herschel ; but he was able to make more repeated 

 measures of this kind, since, on account of the comparatively small power of 

 the instrument, the description of the objects was of secondary importance. 

 It should be remarked that all these measures were taken with the ring- 

 micrometer, no mere estimations being admitted except when they are 

 expressly mentioned. The results derived from each night's observations are 

 given separately. The places given in the catalogues of Sir William and 



