Observations on the Tails of Halley’s Comet. 143 
The want of any satisfactory theory in relation to the tails of 
comets, and the changes which this particular one has undergone, 
render it more desirable to multiply exact observations of its appa- 
rent length, with an account of those circumstances by which it may 
have been affected. 
The apparent length of this appendage depends, Ist, on its abso- 
jute length ; 2d, on its distance from the earth ; 3d, on its intrinsic 
brightness ; for, as this fades away insensibly, the tail will appear to 
terminate where its light is too faint to make a sensible impression 
on the retina ; 4th, on the brightness of the surrounding sky, in con- 
sequence of the illumination of the atmosphere by other light, as 
that of the sun or moon, which weakens the impression made by the 
light of the tail; 5th, on the altitude of the comet, and the opacity 
of the atmosphere, which intercepts and reflects more or less of this 
light ; 6th, on the position of the optic axis; for although the figure 
and color of a bright object can be determined with more precision 
when the optic axis is directed towards it, the existence of a faintly 
luminous one can be more readily detected, and consequently the 
extent of one, whose brightness progressively diminishes from one 
extremity to the other, till it vanishes, can be more correctly deter- 
mined by oblique or indirect vision, and when the optic axis makes 
a considerable angle with the visual ray of the object; and in all 
comparative estimates of its length, as seen at different times, or by 
different observers, it is necessary to know in which of these modes 
it was viewed. Lastly, telescopic vision, in which the field is com- 
paratively of small extent, is necessarily direct ; but the magnitude 
of a faint object will vary, not only with the magnifying powers, but 
with the diameters of the object glasses, provided they have propor- 
tional apertures. 
is one rare phenomenon, which, whenever it is presented, 
claims particular attention, viz. the second tai). 1 have hitherto al- 
luded to the ordinary and proper one, which is nearly opposite the 
sun, and concerning the physical constitution of which the hypothe- 
ses have been numerous, but unsatisfactory. If it is owing to the 
atmosphere of the comet, driven off by the impulse of the sun’s rays, 
how shall we account for several co-existent tails, some of them ta- 
king a very different direction? There is probably no phenome- 
non, which is destined in the progress of observation, to throw more 
light upon the physical constitution and rotatory motion of comets, 
than that of these supernumerary tails. The observation of some 
