SECT, xxxvi.] ENVELOPES OF COMETS. 403 



at prodigious distances from the earth, the solid parts of the 

 nuclei 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 ; un- 

 fortunately 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 re- 

 solved 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 dia- 

 phanous 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 sepa- 

 rated by dark intervals, but 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 were 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, amd was observed in all its 

 changes by Sir William Herschel and M. Gibers. To the 



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