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, 1st, on its abso- 

 lute 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. 



There is one rare phenomenon, which, whenever it is presented, 

 claims particular attention, viz. the second tail. I 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 



