Observations on the Tails of Halley's Comet. 145 



In the following observations, the largest telescope employed, 

 was a five feet achromatic of Dollond's manufacture ; magnifying 

 power from 57 to 260 ; diameter of object glass, 3| inches. As it is 

 not furnished with a micrometer, the magnitudes, positions, and forms 

 of telescopic objects are to be considered as stated only approxi- 

 mately. 



The smallest telescope, (except the finder,) was a portable tele- 

 scope, having a magnifying power of 6.8 ; diameter of object glass, 

 2.6 inches ; focal length, two feet three inches. 



I shall, however, confine myself chiefly to an account of such 

 observations as could be made with most advantage by simple vision, 

 direct, and indirect; and more particularly to the length of the 

 train. As this was generally more conspicuous in October, I shall 

 confine the account to that month, although the train was afterwards 

 seen. The following is a copy of my journal. 



Observations. — Oct. 4, Ah. 15m. A. M. mean solar time. The 

 tail of Halley's comet was distinctly seen with the naked eye, by indi- 

 rect vision, but was invisible when the optic axis was directed to- 

 wards it. It appeared to be nearly opposite to the Sun, but as it 

 was very short, its position could not be satisfactorily determined. 

 Its length, as seen indirectly, was equal to two or three diameters of 

 the head. The apparent magnitude of the latter, as seen with the 

 naked eye, was about equal to that of a star of the first magnitude, 

 as dilated by irradiation, though its brightness did not exceed one 

 of the third or fourth. The tail could only be seen in the absence 

 of all foreign light, except that of the stars, though the head was 

 visible to the naked eye, till within half an hour of Sun rise. With 

 the five feet telescope, and lowest magnifying power, no tail could 

 be seen, but only a rounded mass, resembling luminous vapor, in- 

 creasing in brightness toward the center, near which the brightness 

 increased so abruptly as to entitle this part to the appellation of a 

 nucleus indistinctly defined. 



Oct. 7. — ^The sky became clear at 5 A. M., and the comet was 

 seen till 5^. 30wz. as it was on the 4th, but no tail was detected. 

 The light of the moon, (it being the day after full moon,) was at first 

 the principal obstacle ; afterwards, the dawn and a haziness con- 

 spired. On the morning of the 4th, when I detected the tail, the 

 observation was made under the most favorable circumstances, i. e. 

 at a considerable altitude, after the setting of the moon, and in a 



Vol. XXXI.--N0. 1. 19 



