NEBULAE. 179 



supposed to be situate, the Milky Way being apparently its outward boundary. Yet 

 besides this province with which we are connected, incalculably vast as it is, perfectly 

 inestimable both in length, breadth, depth, and height, there are other provinces within 

 view, equally as capacious, distinct firmaments or clusters, scattered through those ter- 

 ritories of the universe that are accessible to our gaze ; and could we be removed to any 

 of them, the whole of that great scheme of existence circumscribed by the Milky Way, 

 might seem compressed into a small globular patch in space, the aspect presented by the 

 nebulae to ourselves. The term nebula, signifying a cloud or mist, is a denomination 

 given to spots of pale light, which are sprinkled in the heavens, a few of which may be 

 detected by the unaided eye. They vary considerably in shape, size, and luminosity; 

 and occur in numbers, which every improvement of the telescope increases. In Halley's 

 time the whole number known amounted to six ; but he naively remarked, " there are 

 undoubtedly more which have not yet come to our knowledge." Messier, the comet 

 ferret, was the first who paid particular attention to these objects. While looking for 

 Halley's comet, in 1758, whose return was then expected, he observed in the neighbourhood 

 of Tauri a whitish light, elongated like the flame of a taper. This led to the production 

 of his list of these objects, containing a hundred and three, the result of his own 

 personal observation. It appeared in the Connaissance des Temps, for the year 1784, 

 an astronomical almanack published by the French Board of Longitude. Herschel 

 discovered upwards of two thousand more, whose places were determined and catalogued 

 by his sister. Sir John Herschel commenced in 1825 and finished in 1833 the most 

 complete catalogue of nebulae, containing his father's results, and five hundred additional 

 objects, the fruits of his own observation. " I have already determined," he remarks, 

 " with as much accuracy as the nature of such observations permits, the places ; and 

 obtained sufficient descriptions of the physical peculiarities, of between two and three 

 thousand of these wonderful objects a great part of them by many repeated observations, 

 and made careful drawings of the most remarkable for their shape, size or structure. 

 Among these are objects so surprising, that I shall earnestly desire to see my observ- 

 ations verified by the powerful instruments (if sufficiently so) which are now become 

 common in the hands of observers." 



The nebulae exhibit very varying appearances. Their chief aspects are shown in the 

 views inserted at the commencement and close of this chapter, as well as in the other 

 diagrams. A popular account of them may be given under the general heads of resolved, 

 resolvable, and a third class of mysterious substances which appear in the heavens, as 

 if advancing towards an ultimate organisation, some on its verge, others without any 

 display of structure, and others in intermediate stages. 



The first class of nebulae are apparently isolated patches of luminous matter, as seen 

 by the naked eye, or through common telescopes, but by instruments of greater power, 



Nebula in Canes Venatici. 

 N 2 



