404 ENVELOPES OF COMETS. [SECT, xxxvi. 



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

 round mass of light, which was resolved into several distinct 

 parts when viewed with a telescope. A very brilliant in- 

 terior circular mass of nebulous matter was surrounded by 

 a black space having a parabolic form, very 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 distinctness of its outline as he increased the magnify- 

 ing 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 con- 

 trary, Sir William Herschel saw three extremely small stars 

 shining clearly in the black space, which was singularly 

 transparent. As the envelopes were formed in succession as 

 the comet approached the sun, Sir William Herschel con- 

 ceived them to be vapours raised by his heat at the sur- 

 face of the nucleus, and suspended round it like a vault or 

 dome by the elastic force of an extensive and highly trans- 

 parent 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 



