45] ROYAL ASTRONOMICAL SOCIETY TO PEOF. H. D'ARREST. 353 



astronomers (Lord Rosse excepted) in this hemisphere, all brought up to the 

 same epoch." 



Sir John Herschel states that a comparison with Dr Auwers' results led 

 him to the detection of several grave errors in his own work which would 

 otherwise have escaped notice, and whose rectification has added materially 

 to its value. 



Sir John Herschel's general catalogue contains the places and descriptions 

 of 125 of the new nebulae discovered by Professor D'Arrest, and reduced by 

 him to the epoch of that catalogue. 



At the end of his own work Professor D'Arrest gives a catalogue of 

 the mean places of his 1942 nebulae, reduced to the epoch 1860 for com- 

 parison with Herschel's general catalogue. He also gives a comparison of his 

 own positions with the places of 223 nebulae contained in the very accurate 

 special catalogue by Schonfeld, which has been already mentioned. 



In the above rapid sketch I have omitted to mention the many excellent 

 descriptions and delineations of particular nebulae which we owe to Mr Lassell, 

 Professors W. C. Bond and G. P. Bond, Mr Mason, Otto von Struve, Padre 

 Secchi, and others. 



I must not terminate this very imperfect account of the principal 

 additions to our knowledge of the Nebulae which have been made in recent 

 years, without referring to the entirely new mode of investigation to which 

 they have been subjected by means of the spectroscope. By observations of 

 this kind, Mr Huggins and others have thrown much additional light on the 

 nature and constitution of these mysterious bodies. Already the spectra of 

 about 140 nebulae have been examined, and the light from many of them 

 has been proved to emanate from glowing gas. This entirely confirms the 

 mature view of Sir William Herschel, viz., that the condition of the luminous 

 matter in many of the nebulae is widely different from its condition in the 

 fixed stars. 



Professor D'Arrest has himself contributed to the spectroscopic obser- 

 vations of the nebulae, and he has made the suggestive remark, that almost 

 all the gaseous nebulae are found either within or near the borders of the 

 Milky Way, and that there is an entire absence of them in the regions near 

 the poles of the galaxy, in which the other nebulae so abound. I believe 

 that a similar remark was made about the same time by Mr Proctor. 



A. 45 



