45] ROYAL ASTRONOMICAL SOCIETY TO PROF. H. D' ARREST. 349 



at the University Observatory of Upsala, with the Steinheil 13-foot refractor, 

 employing a parallel wire-micrometer with bright spider-lines on a dark field. 



By means of the various series of observations to which I have referred, 

 future astronomers will be provided with a rich store of materials for the 

 study of the proper motions of the nebulas, and we may hope that even in 

 our own time some valuable results may be arrived at respecting them. 



Professor D'Arrest's observations of nebulae were interrupted for a time 

 by his appointment as Director of the Observatory of Copenhagen. In no 

 long tune, however, his new position gave him the opportunity of resuming 

 his observations with the aid of greatly increased optical power. In the year 

 1861, the Observatory acquired a magnificent refractor, by Merz, of 15 feet 

 focal length and 10J French inches in aperture, of which Professor D' Arrest 

 has given an elaborate description in a separate publication, De Instmmento 

 magno cequatorio. He considers this instrument to be intermediate, as regards 

 optical power, between Sir John Herschel's 20-foot reflector in its best con- 

 dition, and the excellent telescope with which Mr Lassell made his observations 

 at Valletta. Finding that with this instrument he could not only perceive 

 the very faintest of the nebulas discovered by the two Herschels, but could 

 make sufficiently precise observations of them, he resolved no longer to 

 continue the work begun in Leipzig, where he confined his attention to 

 selected nebula3, but to enlarge his plan of operations and make a survey 

 of the nebulas of the whole of the northern heavens. At first, indeed, it 

 was his intention to observe all the nebulas he should meet with, whether 

 previously known or not, with the utmost attainable precision, and that not 

 once or twice only but repeatedly. He soon found, however, that to carry 

 out such a plan, especially in such a climate, was beyond human powers, 

 the number of the nebulae far exceeding all expectation. After labouring 

 assiduously and perseveringly at these observations for more than six years, 

 Professor D' Arrest was at length compelled by failing health to bring his 

 work to a close. He estimates that in those six years he had not been able 

 to make more than about one-eighth of the total number of observations 

 which would be required in order to form a catalogue of the approximate 

 positions of those nebulas which could be accurately observed with the Copen- 

 hagen refractor. 



The results of these prolonged labours have been published in the great 

 work, Siderum Nebulosorum Observations Havnienses, 1867. This volume 

 contains about 4800 single positions of 1942 different nebulas. Of these 



