COMETS. 89 



The occultation of the fixed stars by the nucleus of a comet, 

 or by its innermost vaporous envelopes, might throw some 

 light* on the physical character of these wonderful bodies ; but 

 we are unfortunately deficient in observations by which we 

 may be assured * that the occultation was perfectly central ; 

 for, as it has already been observed, the parts of the envelope 

 contiguous to the nucleus are alternately composed of layers 

 of dense or very attenuated vapour. On the other hand, the 

 carefully conducted measurements of Bessel prove, beyond all 

 doubt, that on the 29th of September, 1835, the light of a 

 star of the tenth magnitude, which was then at a distance of 

 7"* 78 from the central point of the head of Halley's comet, 

 passed through very dense nebulous matter, without experi- 



periods of revolution. (Aristot., i. 6, 2.) This Pythagorean doctrine, which, 

 according to the testimony of Apollonius Myndius, was still more 

 ancient, having originated with the Chaldeans, passed over to the 

 Romans, who in this instance, as was their usual practice, were merely 

 the copiers of others. The Myndian philosopher describes the path of 

 comets as directed towards the upper and remote regions of heaven. 

 Hence Seneca says, in his Nat. Qucest., vii. 17 : " Cometes non est species 

 falsa, sed proprium sidus sicut solis et lunce, : altiora mundi secat et 

 tune demum apparet quum in imum cursum sui venit ; " and again, 

 (at vii. 27,) " Cometes aternos esse et sortis ejusdem, cujus caetera 

 (sidera), etiamsi faciem illis non habent similem." Pliny (ii. 25) 

 also refers to Apollonius Myndius, when he says : " Sunt qui et hcec 

 sidera perpetua esse credant suoque ambitu ire, sed non nisi relicta a 

 &*le cerni." 



* Gibers, in Ast. Nachr., 1828, s. 157, 184. Arago, De la Constitution 

 physique des Cometes ; Annuaire de 1832, p. 203, 208. The ancients 

 were struck by the phenomenon that it was possible to see through 

 comets as through a flame. The earliest eviden e to be met with of 

 stars having been seen through comets, is that of Democritus, (Aristot., 

 Meteor., i. 6, 11,) and the statement leads Aristotle to make the not 

 unimportant remark, that he himself had observed the occultation of 

 one of the stars of Gemini by Jupiter. Seneca only speaks decidedly 

 of the transparence of the tail of comets. " We may see," says he, 

 "stars through a comet as through a cloud (Nat. Qucest., vii. 18) ; but 

 we can only see through the rays of the tail, and not through the body 

 of the comet itself: non in ea parte qua sidus ipsum est spissi et 

 eolidi ignis, sed qua rarus splendor occurrit et in crines dispergitur. 

 Per intervalla ignium, non per ipsos, vides" (vii. 26.) The last remark 

 is unnecessary, since, as Galileo observed in the Saggiatore ( Letter a a 

 Mornignor Cesarmi, 1619), we can certainly see through a flame when 

 it u _iot of too great a thickness. 



