158 Dr. Herschel’s Observations of a Comet , 
the eye. Now, as in this case, we are sure of the bright in- 
terposition of the parts of the comet, but have no knowledge 
of floating particles, we ought certainly, not to ascribe an 
effect to an hypothetical cause, when the existence of one, 
quite sufficient to explain the phenomena, is evident. 
If we admit that the observed full illumination of the disk 
of the comet cannot be accounted for from reflection, we may 
draw the same conclusion, with respect to the brightness of 
the head, coma, and tail, from the following consideration. 
The observation of the 2d of February mentions that not only 
the head and coma were still very bright, but that also the 
faint remains of the tail were still visible; but the distance of 
the comet from the earth, at the time of observation, was 
nearly 240 millions of miles,* * * § which proves, I think, that no 
light reflected from floating particles could possibly have 
reached the eye, without supposing the number, extent, and 
density of these particles, far greater than what can be ad- 
mitted. 
My last observation of the comet, on the 21st of February, 
gives additional support to what has been said ; for at the 
time of this observation, the comet was almost 2,9 times 
the mean distance of the sun from the earth. -f- It was also 
nearly 2,7 from the the sun.£ What chance then could rays 
going to the comet from the sun, at such a distance, have to 
be seen after reflection, by an eye placed at more than 275 
millions of miles § from the comet ? And yet the instant the 
* 2 39 8 94939 - 
f The sun’s mean distance being i, that of the comet was 2,89797. 
| The comet’s distance from the sun was 2,68,3196. 
§ 275077889. 
