136 HINTS TO OBSERVERS OF IIALLEY's COMET. 



observed it, and " la figure en croissant est nettemeni dessinde dans les re- 

 gistres de I'Observatoire.* Hevelius also observed, but only on one night, 

 (Sept. 8, six days before the perihelion) a brilliant hook extending itself from 

 the nucleus, in the direction of the tail, for a distance equal to the diameter 

 of the head.i- This could hardly have been a true phasis ; but the possible 

 recurrence of such a remarkable phenomenon deserves to be carefully watch- 

 ed : perhaps this hook was actually what is described as a crescent by the Pa- 

 ris observers. 



€. Whether it may be possible, if there is anything like a planetary disc, 

 ^0 observe the occultation of a fixed star by it ; and, if so, whether it is a to- 

 tal obscuration of the star, or only such a diminution of its light as must ne- 

 jcessarily result from its being viewed, as it were, upon a bright ground. 



7- If there is nothing but a luminous point in the centre, whether this 

 point is constantly visible every night, and during the whole of each obser- 

 vation. 



8. If there should be merely a general condensation of light towards the 

 centre of the head, whether this condensation is sudden or gradual ; and whe- 

 ther it is equally gradual at difi^erent times, or, by its more sudden increase 

 of brightness, and more defined outline at certain periods, affords a suspicion 

 of an interior body obscured by a variable atmosphere. 



9. Whether the central parts or nucleus of the Comet is subject to any re- 

 Jnarkable variations in brightness from night to night, the air continuing 

 equally clear; and whether anything like a periodical return can be detected 

 in such variations. 



10. Whether the nucleus, be it what it may, is situated in the centre of 

 the coma, and whether there is any variation in this respect. In 1682 Flam- 

 steed observed the nucleus of this Comet to be twice as near to the edge of 

 the coma on Sept. 4, as it had been three or four days previously.^! 



11. Whether the coma exhibits an uniform appearance on every side, or is 

 more dense in any direction, or appears as if scattered and interrupted. 



12. Whether the extension of the coma decreases, in proportion to the size 

 .^f the nucleus, as it approaches to the perihelion ; allowance being made for 

 Us increasing distance from the earth, and for the effect of twilight. 



13. Whether there is any dark space included within the head, and sur- 

 rounding the nucleus.; or whether there may be more than one such interval, 

 and whether it, or they, extend round the whole, or only a portion of the 

 nucleus. 



14. Whether the darkness of such included space approximates to the depth 

 -of colour of the exterior sky, and whether this darkness is variable in inten- 

 sity upon different nights. 



15. In 1456, three days before the perihelion, the head of the Comet, view- 

 ed (of course) with the naked eye, shone with the brilliancy and twinkling of 

 a fixed star, so that many persons imagined it must have been in conjunction 

 with several small stars.§ As the present position of the earth with respect 

 to the Comet is far less favourable than during that most splendid of its re- 

 corded appearances, nothing of this kind can perhaps be expected, but it 

 should at least be borne in remembrance, and attended to. 



16. In 1607, when the relative positions of the earth and Comet were pe- 

 culiarly similar to the present, and therefore a similarity of aspect may be 

 expected, the head appeared, to the naked eye, not exactly circular, but, as 

 Kepler expresses it, " quodam modo strumofa, deficiens a rotunditate."|| 



17. Thg length and breadth of the tail should be carefully observed on 

 every favourable opportunity. Much will, in this case, depend upon the 

 state ot the atmosphere and the eye of the observer. In 1456 the tail was 

 not less than 60^ in length. Picard estimated it at 30°, in 1682 ; while He- 



■• Abrege dfAstronomie, p 561. 



+ Littrow, Beytrage, &c., ubi supra. 



t Historia Ceelestis, L. i, P iii, p 110. 



} Littrow, Beytrage, &c., p 20. 



H Idem, p. 36. 



