300 HERSCHEL. 



tail was not flat as it appeared to be ; it had the form of 

 a conoid, with its sides of a certain thickness. The 

 visual lines which traversed those sides almost tangenti- 

 ally, evidently met much more matter than the visual 

 lines passing across. This maximum of matter could not 

 fail of being represented by a maximum of light. 



The luminous semi-ring floated ; it appeared one day 

 to be suspended in the diaphanous atmosphere by which 

 the head of the comet was surrounded, at a distance of 

 518,000 kilometres (322,000 English miles) from the 

 nucleus. 



This distance was not constant. The matter of the 

 semi-annular envelop seemed even to be precipitated 

 by slow degrees through the diaphanous atmosphere ; 

 finally it reached the nucleus ; the earlier appearances 

 vanished ; the comet was reduced to a globular nebula. 



During its period of dissolution, the ring appeared 

 sometimes to have several branches. 



The luminous shreds of the tail seemed to undergo 

 rapid, frequent, and considerable variations of length. 

 Herschel discerned symptoms of a movement of rotation 

 both in the comet and in its tail. This rotatory motion 

 carried unequal shreds from the centre towards the bor 

 der, and reciprocally. On looking from time to time at 

 the same region of the tail, at the border, for example, 

 sensible changes of length must have been perceptible, 

 which however had no reality in them. Herschel 

 thought, as I have already said, that the beautiful comet 

 of 1811, and that of 1807, were self-luminous. The 

 second comet of 1811 appeared to him to shine only by 

 borrowed light. It must be acknowledged that these 

 conjectures did not rest on any thing demonstrative. 



In attentively comparing the comet of 1807 with the 



