COMETS. 



sometimes presents the appearance of a planetary disc seen 

 through a nebulous haze. In general, however, on examining 

 the object with high optical power, these appearances are changed, 

 and the object seems to be a mere mass of illuminated vapour 

 from its borders to its centre. 



78. When a nucleus R apparent, or supposed to be so, the 

 nebulous haze which surrounds it and forms the exterior part of 

 the head is called the coma. 



79. These designations are taken from the Greek word Ko/jL-f] 

 (kome) hair, the nebulous matter composing the coma and tail 

 being supposed to resemble hair, and the object being therefore 

 called KofjL-fjr-rjs (kometes), a hairy star. 



80. As the brightness of the coma gradually fades away towards 

 the edges, it is impossible to determine with any great degree of 

 precision its real dimensions. These, however, are obviously 

 subject to enormous variation, not only in different comets com- 

 pared one with another, but even in the same comet during the 

 interval of a single perihelion passage. The greatest of those 

 which have been submitted to micrometrical measurement was 

 the great comet of 1811, the diameter of the head of which was 

 found to be not less than 1| millions of miles, which would give 

 a volume greater than that of the sun in the ratio of about 2 to 1 . 

 The diameter of the head of Halley's comet when departing from 

 the sun, in 1836, at one time measured 357000 miles, giving a 

 volume more than sixty times that of Jupiter. These are, how- 

 ever, the greatest dimensions which have been observed in this 

 class of objects, the diameter rarely exceeding 200000 miles, and 

 being generally less than 100000. 



81. Attempts have been made where nuclei were perceivable, 

 to estimate their magnitude, and diameters have been assigned to 

 them, varying from 100 to 5000 miles. For the reasons, how- 

 ever, already explained, these results must be regarded as very 

 doubtful. 



Those who deny the existence of solid matter within the coma, 

 maintain that even the most brilliant and conspicuous of those 

 bodies, and those which have presented the strongest resemblance 

 to planets, are more or less transparent. It might be supposed 

 that a fact so simple as this, in this age of astronomical activity, 

 could not remain doubtful ; but it must be considered, that the 

 combination of circumstances which alone would test such a ques- 

 tion, is of rare occurrence. It would be necessary that the centre 

 of the head of the comet, although very small, should pass 

 critically over a star, in order to ascertain whether such star is 

 visible through it. With comets having extensive comae without 

 nuclei, this has sometimes occurred ; but we have not had such 



