148 Observations on the Tails of Hallei/s Comet. 



twice that breadth at its remote extremity. The opposite sides were 

 inclined at an angle of about 20° ; i. e. considering it as a frustum 

 of a cone, the angle at the imaginary vertex was about twenty de- 

 grees. 



Although with such a telescope there can be but little irradiation, 

 compared with that in simple vision, I at the first moment suspected 

 that it might possibly be an illusion of that kind ; but having ob- 

 served, as was stated in my memoir on that subject,* that the direc- 

 tions of the beams produced by this cause, have certain determinate 

 positions with respect to the position of the head, and are conse- 

 quently changed with its inclinations, I soon discovered that this 

 brush of light was not in any one of the directions of maximum ir- 

 radiation ; and on my inclining the head to the right and left, the 

 position of the beam, which I now concluded to be a real tail, re- 

 mained constant. But for more perfect assurance, I removed the 

 day tube, which had been hitherto employed, and which showed 

 objects erect, and applied in succession two other eye pieces, both 

 of which inverted, and one of them was of the highest magnifying 

 power, i. e. 260. The new tail was seen with these glasses with 

 no less distinctness, but appeared to be exactly in the opposite di- 

 rection, just as it should be if real. The object glass was also rota- 

 ted, without producing any change in the tail. The foregoing ob- 

 servations were completed about the time of the rising of the moon, 

 more than half of whose visible disk was, at that time in the month, 

 illuminated. About an hour and a quarter after it had risen, the 

 comet, as seen with the naked eye, presented but a very short tail ; 

 its long one being, as it were, shorn off by the moonlight, to a length 

 equal only to three times the diameter of the head ; i. e. about the 

 same length, compared with the diameter of the nebulous envelope, 

 as that of the telescopic short one, compared with the diameter of 

 the telescopic nucleus. 



Oct. 13. — The atmosphere during the day and evening has been 

 smoky. This opacity has prevented a good view of the comet this 

 evening. The tail, as seen with the naked eye, appears not more 

 than 3° in length. The eye-piece for land objects gives no distinct 

 view through the smoke, on account of the number of glasses. 

 With the lowest magnifying power of the others, which invert, the 

 nucleus appears situated as last night, in the lower and right part of 



* Transactions of the American Philosophical Society, vol. iv. new series. 



