374 ENVELOPES OF COMETS. SECT. XXXV. 



appear like mere points of light, so minute that it is impossible 

 to measure them with any kind of accuracy, so that the best 

 astronomers often differ in the estimation of their size by one- 

 half of the whole diameter. The transit of a comet across the 

 sun would afford the best information with regard to the nature 

 of the nuclei. It was computed that such an event was to take 

 place in the year 1827 ; unfortunately the sun was hid by clouds 

 from the British astronomers, but it was examined at Viviers and 

 at Marseilles at the time the comet must have been projected on 

 its disc, but no spot or cloud was to be seen, so that it must have 

 had no solid part whatever. The nuclei of many comets which 

 seemed solid and brilliant to the naked eye have been resolved 

 into mere vapour by telescopes of high powers ; in Halley's comet 

 there was no solid part at all. 



The nebulosity immediately round the nucleus is so diaphanous, 

 that it gives little light ; but at a small distance the nebulous 

 matter becomes suddenly brilliant, so as to look like a bright 

 ring round the body. Sometimes there are two or three of these 

 luminous concentric rings separated by dark intervals, Jbut they 

 are generally incomplete on the part next the tail. 



These annular appearances are an optical effect, arising from a 

 succession of envelopes of the nebulous matter with intervals 

 between them, of which the first is sometimes in contact with 

 the nucleus and sometimes not. The thickness of these bright 

 diaphanous coatings in the comets of 1799 and 1807 was about 

 7000 and 10,000 leagues respectively ; and in the first comet of 

 1811 the luminous ring was 8000 leagues thick, and the distance 

 between its interior surface and the centre of the head was 10,000 

 leagues. The latter comet was by much the most brilliant that 

 has been seen in modern 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 into several distinct parts when 

 viewed with a telescope. A very brilliant interior 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 



