130 ON COMETS. 



its separation, behind the head, into two main streams 

 with comparative darkness between them. This would 

 be a natural and necessary optical consequence of the 

 tail consisting of a hollow, conical envelope, streaming 

 off on all sides around from the head, and presenting to 

 the eye therefore a much greater thickness of luminous 

 matter at its edges than at its middle. Eut in this comet 

 the separation, when viewed througii powerful telescopes, 

 was singularly sharp ; and appeared as a clear, narrow, 

 straight cut, or dark chink, originating close to the nucleus 

 (as, indeed, on that explanation of the fact it ought). 

 And this brings me to treat of the appearances presented 

 by the head and nucleus under the inspection of power- 

 ful telescopes. 



(45.) All considerable comets which have been ex- 

 amined with anything like what would in these days be re- 

 garded as a/<?z/<?;y/// telescope, have presented the appear- 

 ance of a nucleus of more or less definable and condensed 

 light, sometimes having a much brighter and almost 

 stellar point in or near its centre, and at sonje distance, 

 in the direction of the sun, a capping of light sometimes 

 quite separated, as if some transparent atmosphere 

 sustained it more frequently connected by those fan- 

 like jets of " flame," such as we have mentioned in the 

 case of Halley's comet, and putting on the aspect of a 

 *' sector," or fan, opening out into a widening arc, and 

 bounded internally by two crescents springing from the 

 nucleus. Donati's comet exhibited this feature in per- 

 fection ; not, however, without striking variations and 

 individual peculiarities. There was the same appearance 



