45] ROYAL ASTRONOMICAL SOCIETY TO PROP. H. D'ARREST. 343 



The latter work also comprises valuable chapters on the apparent distri- 

 bution of the nebulae over the heavens, and on their classification, together 

 with many general remarks on the phenomena presented by them, which 

 have been suggested by the author's long experience. 



By these labours of Sir William and Sir John Herschel, and by them 

 almost exclusively, astronomers had now obtained a considerable amount of 

 knowledge respecting the apparent distribution of the nebulae over the 

 heavens, and respecting their forms and physical structure as seen through 

 powerful telescopes. 



Their distances from us, however, and therefore their real distribution 

 in space and their actual magnitudes remained matter of speculation only. 



Sir William Herschel, having found that many nebulae, which in inferior 

 instruments shewed no traces of stellar composition, were, when viewed by 

 his powerful telescopes, resolved entirely into stars, was at first inclined to 

 believe that all nebulae were so resolvable. Hence he was inclined to 

 regard them as so many galaxies, similar in their nature to our Milky Way, 

 and owing their nebulous appearance to the enormously greater distances 

 from us at which they were situated. Longer experience, however, induced 

 him completely to change his views. 



Already in 1791, in a paper on Nebulous Stars, he had arrived at the 

 conclusion that there exists a diffused self-luminous matter "in a state of 

 modification very different from the construction of a sun or star," and that 

 a nebulous star is one " which is involved in a shining fluid of a nature 

 totally unknown to us," and "which seems more fit to produce a star by 

 its condensation than to depend on the star for its existence." 



Again, in his paper on the Construction of the Heavens, in the Philo- 

 sophical Transactions for 1811, he shews that although the appearances 

 presented by diffused nebulous matter and by a star are so totally dissimilar, 

 yet that these extremes may be connected by a series of such nearly allied 

 intermediate steps as to make it highly probable that every succeeding 

 state of the nebulous matter is the result of the action of gravitation upon 

 it while in a foregoing one, and that by such steps the successive conden- 

 sation of it has been brought up to the condition of planetary nebulas, 

 and from this again to a stellar form. 



From the appearances presented by the planetary nebulae he infers that 

 the nebulous matter is partially opaque, since the superficial lustre which 



