150 Observations on the Tails of Halley's Comet. 



Oct. 16, 7 P. M. — This evening the sTcy is dear, and the comet 

 is seen very distinctly. The condensed nebulous matter which ra- 

 diates from the comet on one side, and which, for the sake of dis- 

 tinction, I call the short tail, is very distinct, and forms a longer 

 and less acute cone than it did when first seen ; its length being 

 equal to about six times the diameter of the nucleus, and its sides 

 diverging at an angle of about 60° ; and outside of these and con- 

 tiguous to them, are fainter portions at an angle of about 90° with 

 each other; so that, considering this tail as a frustum of a cone, 

 the parts within 30° of the axis are bright ; beyond that the tail is 

 faint, yet visible to the extent of 45° on each side of the axis of 

 the cone. These are to be understood as the observed angles ; the 

 actual angles being somewhat less, if the axis of the cone is oblique 

 to the visual ray. This cone seems, when viewed with the large tel- 

 escope, to constitute nearly all the envelope then visible. It would 

 seem that the outermost envelope seen with the naked eye, is not 

 seen with this telescope, because it is too faintly luminous, and that 

 this short fan-like tail is not seen with the naked eye, because it has 

 not sufficient magnitude. I infer this want of identity in the visible 

 envelope in the two cases, not merely from the dissimilarity in shape, 

 but from their relative apparent magnitudes ; for the telescopic, con- 

 centrated and radiated envelope, appears, (I should judge,) even 

 smaller than the whole head, as seen with the naked eye. Had it 

 appeared magnified by the telescope, we might have attributed the 

 difference of shape in part to irradiation, most of which would be 

 removed by the telescope. 



At 8 o'clock, the axis of this conical tail, or the middle radius of 

 its projection, considered as a circular sector, is directed downwards, 

 and to the right, making an angle of about 161° (as I judged) with 

 the proper tail. The latter passed exactly through the star & in the 

 knee of Hercules. The small tail was seen with all the eye glasses, 

 also with the tube of the telescope inverted. It was seen by many 

 of the students, as well as by myself. The long tail, as seen with 

 the naked eye, did not appear to increase in breadth for more than 

 three or four degrees from the head of the comet, and the part seen 

 constantly by direct vision, may have been 7° or 8° in length ; 

 but seen by indirect vision, it extended forty five degrees in length, 

 stretching across the heavens nearly to the milky way. 



Oct. 17. — The tail, seen directly, this evening, is usually about 

 3° in length ; its width at that distance is about twice as great as 



