Notices respecting Nexv Books. 



387 



objects physically nebulous and objects only optically so. An object 

 really composed of discrete stars may appear nebulous either because 

 it consists of stars so small as to be separately indiscernible by the 

 light of the telescope, or of stars so close as to be incapable of being 

 separated by its defining and magnifying powers, and under different 

 degrees of instrumental imperfection such an object may offer any 

 variety of appearance." Hence the distinction between nebulae pro- 

 perly so called, and those which we are to consider as certainly or 

 very probably clusters of stars, can never become a permanent ground 

 of classification. Their degree of resolvability, however, connected 

 as it is with the absolute brightness of their constituent stars, and 

 their distance from us, must always form an important character in 

 their description. He further observes that when sensible objects 

 possess no qualities but such as are common to them all, and differ 

 only in the greater or less degree in which those qualities are present 

 in them, the only classification they admit is a classificatio pei- gradus. 

 From these considerations he considers it sufficient to divide the 

 whole nebulous system into three great classes : — I. Regular Nebulae; 

 II. Irregular Nebulae; III. Irregular Clusters. Those of the first 

 class are described under five different characters, and in each cha- 

 racter he recognises five degrees, — two extreme and three medial. 

 The characters and degrees forming the system of sub- classification, 

 are arranged in a particular order as follows : — 



By giving to the first five numerical digits an absolute value de- 

 noting the degree, and a value dejjending on position denoting the 

 character, the description of the nebula is rendered very compendious. 

 Thus the combination I. 3215.5 expresses "A Regular Nebula, 

 Middle-sized, Bright, Circular, Discoid, Milky." The utility of this 

 system, he remarks, " in affording in a very small compass a good 

 deal of information respecting the physical (or optical) characters of 

 a nebula, in thus rendering possible a general descriptive catalogue 

 of convenient magnitude for reference, will, I am disposed to think, 

 be found considerable." The sub-classification, it will be observed, 

 is founded entirely on the optical aspect of the nebulae, without 

 reference to any notions we may entertain of their intimate nature. 



This first chapter concludes with a description of the Magellanic 

 clouds — the Nubeculae Major and IMinor. The general appearance 

 of the two objects to the njvked eye, on a clear night, and in the 

 absence of tlie light of the moon, is described to be " that of pretty 

 conspicuous nebulous patches of about the same intensity with some 

 of the brighter portions of the Milky Way." In a paper published 

 in the Philosophical Transactions for 1828, representations, which 

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