366 



Marvels of the Universe 



TELESCOPIC COMETS. 



WilKout nucleus. With a nucleus. 



When a Comet can only be seen through a 

 telescope it has no tail. Comets visible without 

 the aid of a telescope always have tails. 



When first discovered by means of a telescope, a Comet 

 is usually a small speck of light, perhaps no bigger than 

 a star of the twelfth magnitude, or even less. As it comes 

 nearer the sun it will grow both in apparent and also in 

 real size, and will present the appearance of a more or less 

 circular patch of grey light, ft may or may not have a 

 central condensation of light, which will give it the appear- 

 ance of a star surrounded by a sort of halo or cloud of 

 light. When such is the case, the star-like condensation is 

 technically called the " nucleus," and the surrounding 

 portion the "coma." The question of a tail then arises. 

 Does the Comet or does it not exhibit a tail ? This is a matter of uncertainty. It may be taken 

 as a general rule that Comets which are only visible in a telescope have no tails, but that the 



Comets visible to the naked eye always have tails. This 

 latter fact accounts for the popular idea that unless a 

 Comet has a tail " the man in the street " does not regard 

 the object as a Comet at all, and takes no interest in it. 

 To the rule just stated, that naked-eye Comets always 

 have tails, I cannot recall a single exception ; but the other 

 part of the rule that telescopic Comets never have tails is 

 not so universally' true. But in the case of a telescopic 

 Comet, whilst there may be no tail, properly so called, 

 visible, it will sometimes happen that a coma with its 

 nucleus acquires an elongated shape, which may or may 

 not indicate that a tail will soon appear. 



The general nature of Comets and their tails is a matter 

 not easily dealt with. Now and again a Comet appears in 

 which it will be noticed that luminous matter is persistently 

 ejected from the nucleus, indicative of action closely re- 

 sembling in appearance, and perhaps in actual nature, the 

 ejection of water from the jet of a fountain under pressure 

 from below. On the other hand, with many Comets there 

 is no appearance of the tail being the result of internal 

 forces ; but the tail simply grows as from nothing, and 

 this perhaps is the more common case, though probably 

 to say that it grows from nothing is quite unphilosophical. 

 At this point, perhaps, the reader will ask the question : " What is a Comet ? " " What is it made 

 of ? " I am bound frankly to confess that we do not know ; that these questions do not admit of 



a straight answer. 



The internal structure of tails is an interesting matter of 

 inquiry. Whilst we can give a reason for their being 

 curved (because, being more ethereal than the heads, they 

 lag behind the more dense heads), we cannot pronounce 

 definitely as to what is the cross section of an average tail. 

 Judging by the fact that its outer margins are more 

 luminous than the centre, it may be inferred that the tail 

 is cylindrical in section. Whilst it cannot be said that we 

 really understand how the tail of a Comet is formed, or 

 why it should be formed, it may at least be conceded that 



RORDAME-QUEMSSET COMET 1893. 



This \vas a particularly beautiful Comet. 

 The light of these celestial bodies is due chiefly 

 to electric discharges. 



DONATI'S COMET. 



This, one of the most brilliant and pi 

 turesque celestial sights ever seen, appeared 



1858, and was visible for 112 days 



