SECT. XXXV. TAILS OF COMETS. 375 



diverging branches, which formed the bifid tail of the comet. Sir 

 William Herschel found that the brilliant interior circular mass 

 lost the distinctness 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 blueish green light, whose 

 intensity decreased gradually, not from the centre, but from an 

 eccentric brilliant speck, supposed to be the truly solid part of the 

 comet. The luminous envelope was of a decided yellow, which 

 contrasted strongly with the greenish tint of the interior nebulous 

 mass. Stars were nearly veiled by the luminous envelope, 

 whilst, on the contrary, Sir William Herschel saw three ex- 

 tremely small stars shining clearly in the black space, which was 

 singularly transparent. As the envelopes were formed in succes- 

 sion as the comet approached the sun, Sir William Herschel con- 

 ceived them to be vapours 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 vanished ; but a new one was formed after it had 

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

 Indeed, comets in general are subject to sudden and violent con- 

 vulsions in their interior, even when far from the sun, which 

 produce changes that are visible at enormous distances, and baffle 

 all attempts at explanation probably arising from electricity, or 

 even causes with which we are unacquainted. The envelopes 

 surrounding the nucleus of the comet on the side next to the 

 sun diverge on the opposite side, where they are prolonged into 

 the form of a hollow cone, which is the tail. Two repulsive 

 forces seem to be concerned in producing this effect ; one from 

 the comet and another from the sun, the latter being the most 

 powerful. The envelopes are nearer the centre of the comet on 

 the side next to the sun, where these forces are opposed to one 

 another ; but on the other side the forces conspire to form the 

 tail, conveying the nebulous particles to enormous distances. 



The lateral edges of the tail reflect more light than the central 

 part, because the line of vision passes through a greater depth of 

 nebulous matter, which produces the effect of two streams some- 

 what like the aurora. Stars shine with undiminished lustre 

 through the central part of the tail, because their rays traverse it 



