HINTS TO OBSERVERS OF HALLE Y*S COMET. 137 



velius made it but 12** on the same ni^ht, and never more than 16**. In 

 \T^6, La Nux, in the clear atmosphere of the Isle of Bourbon, traced it for 

 47*'.* 



18. Whether the tail progressively increases in breadth to the extremity, 

 or, on the contrary, assumes a tapering form. 



19. Whether one side of the tail is better defined than the other, as was 

 distinctly observed by Flamsteed in 1682-|-, and whether this appearance is 

 constant. 



20. Whether the tail is curved, and whether it deviates, at its origin, from 

 exact opposition to the sun ; and whether there are any symptoms of a re- 

 verse or reflex curvature at its extremity. 



21. Whether the tail, when near the horizon, vanishes entirely, as if ob- 

 scured by a cloud, as is asserted by Apian to have been the case in 1539.J 



22. Whether the tail is divided lengthwise into two or more streams by a 

 dark space ; whether such space is visible in the neighbourhood of the nucle- 

 us, or appears uninterruptedly to surround it ; whether its depth of shade is 

 uniform ; and whether it extends to the extremity of the tail. 



23. Whether any smaller lateral streams accompany the principal tail in 

 the vicinity of the head. 



24. If there should be several branches in the tail, whether they are all of 

 equal brightness, and whether the difference in this respect is constant ; whe- 

 ther their relative length and breadth remain unaltered ; and whether there 

 is anything like an interruption anywhere in their length. 



25. Whether the light of the tail, or of its several sub-divisions, is uni- 

 formly distributed in the direction of its breadth ; or whether it is most vivid 

 at the edges, or in any other part. 



26. If there should be anything remarkable in the form of the tail, whether 

 the periodical recurrence of that peculiarity may justly lead to the idea of a 

 rotation. 



27- Whether there are any traces of an anomalous tail, L e., a stream of 

 light in the opposite direction, tending towards the sun. 



28. In 1456, the tail was said to have resembled at times a sparkling flame, 

 which gave rise to the conjecture that it might be in conjunction with nume- 

 rous small stars. Such a phenomenon may possibly, at least, recur. 



29. It is peculiarly deserving of attention that in 1607, to which return, 

 as has been already stated, the present bears the nearest resemblance, the 

 tail exhibited that A^ery peculiar, but unquestionably authenticated, appear- 

 ance of coruscations or fluctuations in length and brightness. Kepler's words 

 are very clear and decisive : " testabantur enim de Cometa anni 1607 omnes 

 qui diligenter ilium fuerunt contemplati, caudam jam brevem, mox in ictu 

 oculi longam apparuisse, non tamen, quod Cardani verba et exempla innuere 

 videntur, (concerning the Comet of 1556) flamma, ut sicdicam, ad latera ex- 

 patiante, sed solum prorsum ruente. Adroque mihi, qui debili visu sum, vix 

 aliter apparebat hsec cauda, quam ubi inclaruit seu promicuit : tunc enim 

 non mediocriter longa apparebat, et oculos evidentes|| movebat."§ If such 

 fluctuations can be observed, it will be very desirable to note, if possible, 

 their frequency, and whether they may not affect the breadth as well as 

 length, of the tail. If there are several distinct branches, it should be care- 

 fully noticed whether the coruscations pervade them all. 



30. The colour of the Comet should be remarked, and whether the different 

 parts exhibit any difference or contrast in this respect. In 1759, the nucleus 

 was said to be whitish, and similar in colour to the planet Venus. •[ 



Such rapid variations have frequently been perceived in some Comets, that 

 even hourly observations have ascertained changes of a very obvious and 



* Idem, pp. 55, 64. 



+ Historia Ceelistis, L. i, P. iii, pp. 109, 110. 



t Littrow, Bfytrage, &c. p. 25. 



II evidenter? 



i De Cometis, Augusfce Vindelicorum, mdcxix, p. 102. 



r Littrow, Beytrag^, &c. p 64. 



