148 



• KNOVs^LEDGE ♦ 



[March 9, 1883. 



defined, but rather a nebulous star, some 4" in diameter ; 

 in front of this at a distance of perhaps HO" was an 

 " envelope," and there was a second ow at a distance of 2' or 

 .'V. They were connected by a pair of eccentric circular arcs, 

 and these arcs, coalcscinf,' with the outer envelope and pro- 

 longed, formed the skeleton of the "wings." Back of the 

 nucleus, traces of the usual dark stripe could be detected. 

 Fig. 2 presents the main features in outline, and every one 



Fig. 2.— Head of Comet bcptfuiLiur iu- 



Lu telescope. 



will notice its close resemblance to Brodie's picture of 

 Coggia's comet as seen on July 13, 1874. (The picture 

 alluded to forms the frontispiece of Chambers's " Descrip- 

 tive Astronomy," third edition.) 



On the next day the comet was seen at Princeton for a 

 few moments through clouds, just long enough to get im- 

 perfect observations for position, but nothing more. It was 

 noticed, however, that the eccentric arcs had disappeared. 

 On October 2 the comet was observed for more than an 

 hour before daybreak with the great telescope. The most 

 notable features were a single bright cap or envelope at a 

 distance of about half a minute from the nucleus, and the 

 nucleus itself, which, instead of being round, was consi- 

 derably elongated. There were, however, no jets, or star- 

 fish-like projections, such as the comet of 1881 presented 

 so often. There was not much of structural detail to be 

 made out in the head of the comet, but the dark stripe 

 behind the nucleus was very conspicuous. This dark stripe, 

 by-the-way, is a very remarkable phenomenon, not yet 

 explained, so far as we know, though observed in most 

 large comets. The common impression is, that it is merely 

 a space behind the nucleus, screened as it were by the 

 nucleus itself, from the rush of luminous matter which is 

 being driven backward by the sun's repulsion. But if 

 this be so, then, as Mr. Proctor has pointed out in a 

 recent article, there is no reason why it should appear so 

 well defined and so dark. The cross-section of the tail, a 

 little way behind the nucleus, was, in the present case at 

 least, 100,000 miles in diameter : now, merely taking 

 away the luminous matter from a tunnel 6,000 or 8,000 

 miles in diameter along the axis of the tail, could make but 

 little difference with the amount of light received by the 

 eye at a distance. If there were no tunnel, we should get 

 from the central line of the tail the brightness corre- 

 sponding to a thickness of 100,000 miles of luminous 

 matter. Boring the tunnel would only reduce it to 

 some 90,000 miles, and the ditterence would be hardly 

 perceptible. 



It seems more likely, if the writer may venture the sug- 

 gestion, that the stripe is a stream or beam of non-luminous, 

 cooler-vapour or gas, which is nearly opaque to the radia- 

 tion emitted by the same kind of gas when luminous, 

 and therefore cuts ofl" all the light from whatever portions 

 of the comet's luminous drapery is behind it ; in the 

 same way that cool sodium-vapour, for instance, is rela- 



tively opaque to the light of a sodium-flame. If this is 



correct, the dark stripe ought not to Vie black, liut just 

 about half as bright as the neighbouring nebulosity ; 

 which corresponds to the actual fact. If one could catch 

 a star passing behind the stripe, it would perhaps be easy 

 to settle the question. At any rate, if the star shone more 

 brightly when in the stripe, we might be sure that the 

 hypothesis is wrong. The star should be dimmed a little, 

 if anything, though, of course, star-light would not be so 

 much affected as the light from cometary matter. Mr. 

 Proctor has suggested a different hypothesis,* which seems 

 to the writer rather less probable, but there is no time to 

 discuss it here. 



On October 4th the nucleus had become much more 

 elongated, so as to be shaped something like an Indian 

 club. The envelope, which was conspicuous on the 2nd, 

 had disappeared, or degenerated into an indefinite nebu- 

 losity, and the dark stripe had become much fainter. 



Continued bad weather prevented observation until the 

 10th, and on this date the nine-and-a-half-inch telescope of 

 the School of Science Observatory was used. A great 

 change had taken place. The nucleus had become an irre- 

 gular, spindle-shaped streak some 40" long, made up of 

 six or eight star-like knots of luminosity connected and 

 veiled by shining haze. One of these knots, about a third 

 of the way from the sunward extremity, was considerably 

 larger and brighter than any of the others, and should, 

 perhaps, be considered as the true nucleus. The next one 

 beyond it (reckoning from the direction of the sun) was 

 second in size, and separated by an interval of 2'' or 3' , 

 the space being filled, however, with nebulosity. The dark 

 stripe was still visible, but directed, not along the prolon- 

 gation of the nuclear streak, but inclined at an angle of 8° 

 or 10°, while a bright jet from the nucleus, two or three 

 minutes in length, touched one side of the dark stripe, and 

 kept nearly in the axis of the tail. 



3.— Head of Comet, Oct. 10, 18S2. 



Fig. 3 is an attempt to illustrate the appearance and 



* 1 believe Prof. Young's theory to be nearer the truth than the 

 one I suggested. But some observations of my own lead me to 

 doubt whether the mere shadow of the nucleus would not suffice to 

 exjilaiii the dark streak (despite my own objections, mentioned 

 above by Prof. Young, to this theory). I over-estimated the dark- 

 ness of the streak, and under-estimated the effect of contrast ou 

 ordinary eyesight. As to this last ]ioint. I made a little observation 

 a few evenings since which may be worth recording. Walking with 

 Jlrs. Proctor on the Brighton Parade late in the evening, when there 

 was a good deal of mist in the air, my attention was called by her 

 (almost at the instant that I had noted the phenomenon myself) to 

 what looked like a long, nearly black streak extending vertically 

 upwards above the nearest street-lamp. Of course, this dark streak 

 was the shadow of the knob en the top of the lamp-case ; but there 

 was plenty of illuminated air both beyond the shadowed region 

 and between that region and ourselves. The loss of light from the 

 long conical region of air in the shadow of the lamp-knob was, in 

 fact, much more obvious than we should have expected it to be. 

 The darkness could only be recognised, we found, when one of the 

 side bars of the lamp-frame hid the flame of the lamp from view. 



