Observations on the Cornet of Halley. - 211 



Coma almost entirely fills the field of view, whose diameter is 38'. 

 The nucleus has much the appearance of one of Jupiter's satellites, 

 but with scarcely any sensible magnitude, Oct. 12th, nucleus dis- 

 tinctly visible to the naked eye — train about six degrees in length, 

 points one degree west of a Draconis — slightly hazy. 



On the 22d of September, I commenced observing with a small 

 equatorial, belonging to the College. It has a telescope of eigh- 

 teen inches focus, with a declination circle, graduated to half de- 

 grees, and reading by a vernier to minutes, manufactured by Banks 

 of London. To prepare for observations, I procured a thin plate of 

 silver, with a rectangular cavity, made as accurately as possible, 

 which I secured firmly in the focus of the instrument, in the place 

 of the parallel wires, so that the field of view became rectangular, 

 instead of circular. The wires were useless, there being no contri- 

 vance for illuminating the field, and indeed most of the time the 

 comet would not bear illumination. 



The several adjustments were made substantially, according to 

 the directions of Vince, and for verification I observed the differences 



• 



of Right Ascension and Declination of two known stars near each 

 other, obtaining a result which differed little from the tables. As 

 the instrument has no micrometer, it was necessary to observe the 

 two Declinations independently of each other. In observing the 

 Right Ascension of the comet, I noted merely the time of egress 

 from the field, as also that of the star with which I was comparing 

 the comet. When it could conveniently be done, I selected for ob- 

 servation, a star upon each side of the comet ; noting the egress of 

 the first star, then the comet, then the second star, repeating the ope- 

 ration half a dozen times or more, until I was satisfied with the re- 

 sult. Whenever the nucleus of the comet was distinctly visible in 

 the equatorial, I observed its egress ; when this was not the case, I 

 noted the egress of the point of greatest brightness, which I could 

 generally determine to a second or two of time. On one evening I 

 noted the times of egress of both sides of the coma, and afterwards 

 took their mean ; but I found I could bisect the comet more accu- 

 rately than I could note the egress of its border. — The clock was reg- 

 ulated to sidereal time, and its error each evening determined by ob- 

 serving the meridional transit of some known star. The following 

 table contains a summary of the observations before the perihelion 

 passage. The differences of Right Ascension and Declination, are 



