350 ADDRESS ON PRESENTING THE GOLD MEDAL OF THE [45 



about 390 have either not been previously observed, or have not had their 

 places determined. Sir John Herschel's Northern Catalogue of Nebulae and 

 Clusters of Stars contains a larger number of objects, viz., about 2300. The 

 difference between these numbers partly arises from the fact that D'Arrest 

 has designedly omitted those objects in Herschel's catalogue which, in his 

 judgment, should not be classed with the nebulae, viz., clusters and collections 

 of stars belonging to Sir William Herschel's sixth, seventh, and eighth classes. 

 These clusters appear to have no necessary connection with true nebulae, and 

 they are distributed over the sphere in a totally different manner. The 

 number of such clusters, especially near the Milky Way, might be easily 

 greatly increased ; and in making his sweeps, Professor D'Arrest has often 

 been surprised to find certain clusters inserted in Herschel's catalogue, while 

 several others in the same neighbourhood were omitted. The selection appears 

 to him arbitrary and by no means natural. He thinks too that the intro- 

 duction of these objects would tend to vitiate any inquiries into the law of 

 distribution of the nebulas. 



By far the greater number of the nebulae cannot be observed at all with 

 bright wires, or at any rate can only be so observed by great expenditure 

 of time and trouble. Hence Professor D'Arrest did not attempt to define 

 their places with all the precision of which his instrument was capable, but 

 brought each nebula into the centre of the ring-micrometer, the smallest 

 radius of which was 3' 40". The power employed in determining all these 

 approximate positions was 123. The hour circle was read off to integral 

 seconds of time, and the declination circle to tenths of a minute of arc. 



In fact, nearly the same method was followed which astronomers are 

 accustomed to employ in finding the places of very faint comets. Thus 

 everything was scrupulously avoided which would interfere with the keen- 

 ness of vision, and the more precise definition of place was generally left to 

 micrometrical observations and comparisons with minute stars situated in the 

 immediate neighbourhood of the nebula. 



The nebulae were generally observed in zones of about 4 or 5 in 

 breadth, and in each zone 4 or 5, or even sometimes 7 fixed stars of the 

 7th or 8th magnitude were included, whose places were taken from Bessel's 

 or Argelander's zones, or sometimes from those of Lalande. 



The work contains about 4000 micrometrical measures, chiefly made with 

 the ring-micrometer. More rarely nebulae were compared with the stars and 

 with each other by means of the wire-micrometer. Bright and small nebulee, 



