340 Dr. Herschel’s Observations of a Comet , 
which indicated th@ absence of the nebulous matter that had 
formerly been lodged there. 
A rotatory motion of the comet, which has been suggested, 
would also explain the frequent variations in the length of the 
opposite branches which inclosed the tail ; for if any portion 
of the cometary matter should be more susceptible of being 
thrown into a luminous decomposition than some others, a 
rotatory motion would bring such more susceptible matter into 
different situations, and cause a more or less copious emission 
of it in different places. 
The additional short and faint double streams of nebulous 
light which issued from the vertex or side of the enfeebled 
envelope, in the gradual regress of the comet, tend likewise 
to add probability to the conception of a rotatory motion ; for 
the changeable appearance of the situation of these streamlets 
might arise from a periodical exposition of some remaining 
small portions of less rarefied matter, when nearly the whole 
of it had been exhausted* 
Of the Result of a Comet's Perihelion Passage . 
After having given a detail of phenomena, and entered into 
a research of the most likely manner in which they were pro- 
duced, I shall only mention what appears to me to be the most 
probable consequence of the perihelion passage of a comet. 
The quality of giving out light, although it may always 
reside in a comet, as it does in the immensity of the nebulous 
matter, which 1 have shown to exist in the heavens, is exceed- 
ingly increased by its approach to the sun. Of this we should 
not be so sensible, if it were not accompanied with an almost 
