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uniform ; for though in some parts their light is uniformly milky, in 

 others it is more faint, and might, at first sight, be thought resolv- 

 able into stars. The greater brightness in one part than in another 

 may arise either from greater depth or from greater density, and is 

 thought by Dr. Herschel rather to arise from the latter, which he as- 

 cribes to a mutual gravitation of its parts to each other. Facts, says 

 the author, are not wanting to prove that such a power has been ex- 

 erted ; and it is his intention to point out a series of phenomena 

 where the vestiges of such exertion may be distinctly seen. 



Those nebulae, which from their present want of uniformity seem 

 to have more than one centre of attraction, may, in fact, at this time 

 consist of two nebulae, the limits of which unite only in appearance ; 

 or they may be actually united, but undergoing a gradual sepa- 

 ration. 



In the same manner other nebulae, for which the author refers to 

 his catalogues, are found to be treble, quadruple, sextuple, &c. 



With regard to the forms of nebulae, some are narrow and very 

 long, others of equal length are in a certain degree broader ; others 

 again are of an irregular figure ; and from all these, nothing can be 

 inferred as to their extent in the third dimension, which is not seen. 

 But when the visible surface is nearly elliptical, and still more if the 

 circumference be nearly circular there, a spheroidical or spherical 

 form may with some confidence be presumed ; and these, it is ob- 

 served, are forms naturally resulting from a principle of attraction. 



The central brightness of some nebulae point out the seat of prin- 

 cipal attraction ; and though in many such a difference is scarcely 

 perceptible, there are many others in which the brightness of the 

 centre is considerably greater than that of their circumference, so as 

 to present very much the appearance of many telescopic comets, of 

 which the nucleus is not distinct ; and others also, in which a cen- 

 tral nucleus is distinctly seen, surrounded by a bright and circular 

 nebula. 



These various degrees of condensation, seen in different nebula?, 

 are supposed by the author successively to take place in the same 

 nebula, and are thus connected into one view, to show the progress 

 of condensation. 



The condensation, says Dr. Herschel, may be considered as evi- 

 dence of their materiality ; and he also infers opacity, which is an- 

 other property of matter, from the circumstance of their not being 

 so bright as he would expect from their observed condensation. 



The appearance of certain very regular nebulas, with extensive 

 branches, suggests to Dr. Herschel various queries respecting the 

 gradual change and ultimate result of condensation. 



Do not the branches connected with a nucleus resemble the zo- 

 diacal light connected with our sun ? 



May not portions of branches collect into a planetary form and re- 

 volve around the central nucleus, having themselves a rotatory mo- 

 tion, in consequence of the inequality and irregular position of dif- 

 ferent branches ? 



