made with a view to investigate Us Magnitude , &c 157 
the case, the deficiency I think would have been perceived, 
notwithstanding the smallness of the object. Those who are 
acquainted with my experiments on small silver globules,* 
will easily admit, that the same telescope, which could shew 
the spherical form of balls, which subtended only a few tenths 
of a second in diameter, would surely not have represented a 
cometary disk as circular, if it had been as deficient as are the 
figures which give the calculated appearances. 
If these remarks are well founded, we are authorised to 
conclude, that the body of the comet on its surface is self- 
luminous, from whatever cause this quality may be derived. 
The vivacity of the light of the comet also, had a much greater 
resemblance to the radiance of the stars, than to the mild re- 
flection of the sun's beams from the moon, which is an addi- 
tional support of our former inference. 
The changes in the brightness of the small stars, when they 
are successively immerged in the tail or coma of the comet, 
or cleared from them, prove evidently, that they are suffici- 
ently dense to obstruct the free passage of star-light. Indeed 
if the tail or coma were composed of particles that reflect the 
light of the sun, to make them visible we ought rather to ex- 
pect, that the number of solid reflecting particles, required 
for this purpose, would entirely prevent our seeing any stars 
through them. But the brightness of the head, coma, and tail 
alone, will sufficiently account for the observed changes, if 
we admit that they shine not by reflection, but by their own 
radiance ; for a faint object projected on a bright ground, or 
seen through it, will certainly appear somewhat fainter, al- 
though its rays should meet with no obstruction in coming to 
* See Phil. Trans, for 1805, page 38, the 5th experiment. 
