34 



NATURE 



\yan. 3, r 



great similarity to Donati's comet with its "concentric enve- 

 lopes." But it did not do so. Envelopes there undoubtedly 

 were, but instead of being concentric they were excentric, and 

 of an entirely unique arrangement. 



To give an idea of the appearance presented by these ex- 

 centric envelopes, still referring to the fan, let us imagine a 

 circle to be struck from the left-hand corner with the right-hand 

 corner as a centre, and make the arc a litt'e longer than the 

 arc of the fan. Do the same with the right-hand corner. Then 

 with a gentle curve connect the end of each arc with a point in 

 the arc of the fan half-way between the centre and the nearest 

 corner. If these complicated operations have been properly 

 performed, the reader will have superadded to the fan two ear- 

 like things (as of an owl), one on each s'.de. Such "ears," as 

 we may for convenience call them, were to be observed in the 

 comet, and they at times were but little dimmer than the fan. 

 It will be observed that there is a central depression between 

 the ears. 



At first it looked as if these ears were the parts of the 

 head furthest from the nucleus in advance along the comet's 



axis, but careful scrutiny revealed, still further forwards, a cloudy 

 mass, the outer surface of which was convex, while the contour 

 j of the inner surface exactly fitted the outer outline of the ears 

 [ and the intervening depression. This mass was at times so faint 

 I as to be almost invisible. But at other times it was brighter 

 than all the oiher details of the comet which remain to be de- 

 scribed, now that I have sketched the groundwork. Occasion- 

 ally to be seen outside all was still another fainter mass, both the 

 j surfaces of which were convex outwards, the inner one having 

 a greater radius. This exterior envelope or "umhuUung" was 

 the faintest part of the head. 



In the root of the excessively complex tail were to be observed 

 prolongations of all ihe curves to which I have referred. Thus, 

 j behind the brightest nucleus was a region of darkness which 

 j opened out 45° or 60°, the left-hand boundary of which was a 

 ! continuation of the lower curve of the right ear. All the bound- 

 j aries of the several different shells which showed themselves, 

 t not in the head in front of the fan, but in the root of the tail 

 t behind the nucleus, were continuous in this way — the boundary 

 of an interior shell on one side f)f the axis bent over in the head 



Fig. 23. — Rough oatline sketch of head and envelopes of Coggia's comet as seen in Mr. Newalfs 2^-ir.ch .-' 



(peiihehon jassage, August 27). 



or on the aight of July 12, 1874 



to form the boundary of an exterior shell on the other side of 

 the axis. 



I next draw attention to the kind of change observed. To 

 speak in the most general terms, any great change in one " ear " 

 was counterbalanced by a change of an opposite character in the 

 other; so that, when one ear was thinned or elongated, the 

 other widened ; when one was dim, the other was bright ; when 

 one was more " pricked " than usual, .he other at timesappeared 

 to lie moie along the curve of the fan and to form part of it. 

 Another kind of change was in the fan itself, especially in the 

 regularity of its curved outline and in the manner in which the 

 straight sides of it were obliterated altogether by light, as it 

 were, streaming down into the tail. 



1 here zvas iioihing xuhich in thi slightest degice rtscmbled the 

 giving off of vafotij-. 



The only constant feature in the comet was the exquisitely 

 soft darkness of the region extending lor some little distance 

 behind the nucleus. Further behind, where the envelopes, the 

 prolongation of which formed the tail, were less marked, the 



delicate veil which was over even the darkest portion became 

 less delicate, and all the features were merged into a mere 

 luminous haze. Here all structure, if it existed, was non- 

 recognizable, in striking contrast with the region round [and 

 immediately behind the fan. 



Next, it lias to be borne in mind that the telescopic object is, 

 after all, only a projection, from which the true figure has to be 

 built up, and it is when this is attempted that- the unique charac- 

 ter of this comet becomes apparent. Tl ere were no jets, there 

 were no concentric envelopes ; but, in place of the latter, excen- 

 tric envelopes ind catcd by the ears and their strange backward 

 curvings, and \ ossibly also by the fan itself. 



It seems impossible that we can be here dealing with the mere 

 volatilization of the materials of which the nucleus is composed ; 

 for, assuming that it is possible, as has hitherto been imagined, 

 that shells of vapours can be thrown oft' to form concentric 

 envelopes, and that the heads of comets like Donati's are thus 

 built up, it is difficult at first to see how such appearances as 

 here described could be thus produced. 



