Cometary Light. 281 



COMETARY LIGHT.— NEBULA.— OCCULTATIONS. 



BY THE EEV. T. W. WEBB, A.M., E.B.A.S. 



It is curious to remark the occasional revival, perhaps in a 

 modified form, of speculations which have for a time been laid 

 aside. At the period when much attention was drawn to the 

 " Nebular Theory " of Laplace, and the condensation of lumin- 

 ous mists into "blazing suns was commonly spoken of as the 

 probable history of the universe, an idea was entertained by 

 Sir W. Herschel that comets might be detached portions of 

 nebulous matter attracted to our sun from the depths of space ; 

 and though his speculation was not announced in terms corres- 

 ponding with the requirements of mathematical accuracy, yet 

 it was in itself sufficiently remarkable, and would be rendered 

 additionally probable by what he did not refer to in this 

 instance, though he himself discovered it — the proper motion 

 of the sun through space, which would be still bringing fresh 

 portions of nebulous material within the sphere of his attrac- 

 tion. The doubt subsequently thrown upon Laplace's theory 

 by the real or supposed resolution of certain nebulae into stars 

 with increased optical power, of course restored the comets to 

 their independent rank. But the researches of later times, 

 without giving countenance to Laplace's speculation — for the 

 materials of the gaseous nebulae do not appear capable of being 

 condensed into suns like our own — have yet tended to re- 

 establish the connection between these two most mysterious 

 classes of bodies. Our readers are acquainted with the great 

 fact that Huggins has obtained evidence of the gaseous nature 

 of the nucleus of TempePs comet (I. 1866) ; or at least of the 

 identity of its constitution with that of the " planetary nebulee" 

 of Jjk This was the first precise intimation that had ever been 

 obtained as to the real composition of these wandering members 

 of our system. It is true that as a spectroscopic investigation 

 it had been anticipated by Donati at Florence, in the case of 

 Comet II. 1864, but his results are more equivocal, as he was 

 less distinct in his interpretation of what he saw ; nor could 

 this be wondered at in any one who neither had the extra- 

 ordinary experience of eye and hand which distinguishes our 

 great observer, nor was previously aware of the peculiarity of 

 the nebular spectra. At first Donati says that he perceived 

 distinctly two dark bands in {he comet's light ; but he adds 

 that the spectrum resembles that of the metals, the obscure 

 being broader than the luminous spaces ; and that it might be 

 equally said to consist of three light bands. Hence, little more, 

 indeed, could be inferred with safety than that it was elementary 



