104 C0SM09. 



The occultat.ion of the fixed stars by the nucleus of a com 

 et, 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 vapor. 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- 

 encing any deflection during its passage.! If such an absence 

 of refracting power must be ascribed to the nucleus of a com- 

 et, tve can scarcely regard the matter composing comets as a 

 gaseous fluid. The question here arises whether this absence 

 of refracting power may not be owing to the extreme tenuity 

 of the fluid ; or does the comet consist of separated particles 

 constituting a cosmical stratum of clouds, which, like the 

 clouds of our atmosphere, that exercise no influence on the 



{sidera), etiamsi faciem illis non habent similemJ''' Pliny (ii., 25) also re- 

 fers to Apollonius Myndius, when he says, "-Sunt qui et kcBC sidera per- 

 petua esse credant sitoque amhitu ire, sed non nisi relicta a sole cerniy 



* Olbers, in Asir. Nachr., 1828, s. 157, 184. Arago, De la Constitu- 

 tion physique des C o metes ; Annuaire de 1832, p. 203, 208. The an- 

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

 through comets as throusrh a flame. The earliest evidence to be met 

 with of stars having been seen throngh comets is that of Democntus 

 (Aristot., Metear., i., 6, 11), and the statement leads Aristotle to make 

 the not miimportant remark, that he himself had observed the occulta- 

 tion of one of the stars of Gemini by Jupiter. Seneca only speaks de- 

 cidedly 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 tlie tail, and not through the 

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

 :^olidi ignis, sed qua varus 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 Saggialore {Lettera a 

 Monsignor CesaHni, 1G19), we can certainly see through a flame when 

 it is not of too great a thickness. 



t Bessel, in the Astron. Nachr., 1836, No. 301, s. 204, 206. Struve, 

 in Recueil des Mim. de V Acad, dc St. Petcrsh., 1836, p. 140, 143, and 

 Astr. Nachr., 1836, No. 303, s. 238, writes as follows: "At Dorpat the 

 star was in conjunction only 2"*2 from the Imghtest point of the comet. 

 The star remained continually visible, and its hght was not perceptibly 

 diminished, while the nucleus of the comet tfeemed to be almost extin 

 guished before the radiance of the small star of the ninth or tenth mag 

 nitude." 



