Observations on Comet II. 209 



Again, in the case of Donates Comet, the nucleus, about a 

 fortnight before the day of its nearest approach to the sun, 

 might be plainly observed to throw out faint rays of light to- 

 wards that luminary. At Eome, on September 16th, 1858, 

 M. Rosa (one of the discoverers of Comet II.) observed two 

 divergent streams of light shot out from the nucleus of Donates 

 Comet. These proceeded for a short distance towards the front 

 of the coma, then abruptly turned backwards and streamed into 

 the tail; and M. Rosa compared them to long hair when brushed 

 upwards from the forehead, and then allowed to fall on each 

 side of the head. Six days later they had given place to a fan- 

 like bright sector (or semicircular disc of light). On the 27th, 

 this " fan" appeared more spread out. On the 30th, the fan 

 still continuing, a new set of phenomena began to appear. A 

 succession of luminous hoods or " envelopes" were observed, 

 like canopies over the nucleus from which they had been 

 emitted, and these too ultimately streamed back into the tail. 



The reader must not suppose that this streaming motion 

 was visible to the eye. Taking into consideration the great 

 distance of the comet, a movement of a thousand miles a day 

 (such as, for instance, was observed in one of those strange 

 <e envelopes" of light) could not be detected as motion, though 

 sufficiently evident in its effects. Thus Bessel, watching Hal- 

 ley's Comet, with unremitting attention during one long night 

 in October, from sunset to sunrise, was rewarded by seeing a 

 jet from the nucleus describe in that time an arc of thirty-six 

 degrees ; and Bond was able to observe the germs of the 

 " envelopes" at the surface of the nucleus, and to trace them 

 through successive stages to their full development. 



Such changes, then, varying in detail, but bearing a con- 

 siderable general resemblance in all bright comets which have 

 been closely watched with the telescope, go on before the eyes 

 of astronomers ; but meanwhile, the cause of them remains a 

 profound mystery ; nay, more, they are seemingly in defiance 

 (says Mr. Bond) of the best established properties of matter, 

 the laws of gravitation and inertia. Here he speaks of the 

 developments observable in the comet's own physical structure. 

 With regard to the motion of comets in space, there the laws of 

 gravitation do hold good ; but, strange to say, it is the nucleus 

 alone which moves in obedience to the attractive force of the 

 sun and planets. "Immense volumes of matter," continues 

 Mr. Bond, "apparently of the identical substance of the nucleus, 

 go to compose the enveloping nebulosity and the tail, but 

 from the moment of leaving the central body, their motion is 

 perfectly inexplicable, without assuming them to be under 

 the influence of laws of force, which greatly modify that of 

 gravitation." 



