the Comet of 12,11, 351 



one body of light ; lb that the diameter of the coma 

 may be taken, for the diameter of the comet itlelf. 

 A telefcope, by expanding the head, often makes it ap- 

 pear lefs bright, and lefs diftincf. It fprcads it out, in- 

 to a kind of nebula, which has no well defined boun- 

 dary ; but which may, notwithftanding, be miflaken 

 for the nucleus. The latter is generaliy too fmall, to 

 be diftin<ftly perceived, by ordinary telefcopes ; and in 

 many inftances, cannot be difcovered, by thofe of the 

 greateft magnifying power. That of IS07, according 

 to Dr. Herschel, fubtended an angle of only one or 

 two feconds ; while the diameter of the coma was two 

 or three hundred times as ereat La Place and others 

 have even advanced the opinion, that the whole body 

 of the comet is fometimes a miCre collecfion of aeri- 

 form fluids, mofl: condenfed near the centre, but con- 

 taining no fubftance fufficiently compa<fl, toobftrucl en- 

 tirely the paflage of light. In the prefent inilance, I 

 obferved nothing, v/hich could either invalidate or con- 

 firm, fuch an hypothelis. The head appeared, like an 

 obfcure cloud or mift. The comia was nearly of the 

 fame brightnefs with the tail. A darker fpace, or a 

 zone of weak light, very perceptibly fainter than either 

 the head, or the coma, intervened between the two. 



The tail of this comet, was more fplendid, than is 

 common : though fome others have exceeded it, both 

 in brightnefs and extent. That of 1680, was two or 

 three degrees in breadth, and about 70, in length. 

 That of 1759 was, according to Fingre, 90° long. One 

 in 1618 is faid to have extended more than a hundred 

 degrees. 



The length cannot be meafured, with exaclnefs. For 

 the extremity does not terminate at once ; but gradu- 

 ally diminiflies in brightnefs, till it is loft, in the IHU fain- 

 ter light of the Iky. The dimenlions M'ill vary, accord- 

 ing to the ftate of the atmofphere. They will even ap- 

 pear different, to different eyes, at the fame time. A- 

 bout the middle of October, I judged the tail of this co- 

 met to be nearly or quite 1 .5 degrees long. It could not 

 be lefs than twelve. If it be taken at 12'-^ on the 17th j 



