SECT. XXXVI. ENVELOPS OF COMETS. 353 



10,000 leagues respectively ; and in the first comet of 

 1611, the luminous ring was 8000 leagues thick, and 

 the distance between its interior surface and the center 

 of the head was 10,000 leagues. The latter comet was 

 by much the most brilliant that has been seen in mod- 

 ern times ; it was first discovered in this country by Mr. 

 James Vietch of Inchbonny, and was observed in all its 

 changes by Sir William Herschel and M. Olbers. To 

 the naked eye, the head had the appearance of an ill- 

 defined round mass of light, which was resolved hi to 

 several distinct parts when viewed with a telescope. 

 A very brilliant interior circular mass of nebulous mat- 

 ter was surrounded by a black space having a parabolic 

 form, veiy distinct from the dark blue of the sky. This 

 dark space was of a very appreciable breadth. Exterior 

 to the black interval there was a luminous parabolic 

 contour of considerable thickness, which was prolonged 

 on each side in two diverging branches, which formed 

 the bifid tail of the comet. Sir William Herschel found 

 that the brilliant interior circular mass lost the distinct- 

 ness of its outline as he increased the magnifying power 

 of the telescope, and presented the appearance of a 

 more and more diffuse mass of greenish or bluish-green 

 light, whose intensity decreased gradually, not from the 

 center, but from an eccentric brilliant speck, supposed 

 to be the trtfly solid part of the comet. The luminous 

 envelop was of a decided yellow, which contrasted 

 strongly with the greenish tint of the interior nebulous 

 mass. Stars were nearly veiled by the luminous en- 

 velop, while, on the contrary, Sir William Herschel saw 

 three extremely small stars shining clearly in the black 

 space, which was singularly transparent. As the en- 

 velop* were formed in succession as the comet ap- 

 proached the sun, Sir William Herschel conceived them 

 to be vapors raised by his heat at the surface of the 

 nucleus, and suspended round it like a vault or dome by 

 the elastic force of an extensive and highly transparent 

 atmosphere. In coming to the sun, the coatings began 

 to form when the comet was as distant as the orbit of 

 Jupiter, and in its return they very soon entirely van- 

 ished ; but a new one was formed after it had retreated 

 as far as the orbit of Mars, which lasted for a few days. 

 23 GG2 



