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. Quaest.', vii., 18); but we can ony 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 solidi ignis, sed qua rarus splendor occurrit et
in crines dispergitur. Per intervalla ignium, non er ipsos, vides" (vii.,
26). The last remark is unnecessary, since, as Galileo observed in the
'Saggiatore (Lettera a Monsignor Cesarini', 1619), we can certainly see
through a flame when it is not of too great a thickness'.
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 experiencing any deflection during its passage.*
[footnote] *Bessel, in the 'Astron. Nachr.', 1836, No. 301, s. 204, 206.
Struve, in 'Recueil des Mem. de l'Acad. de St. Peterab.', 1836, p. 140, 143,
and 'Astr. Nachr.', 1836, No. 303, s. 238, writes as follows: "At Dorpat
the star was in conjunction only 2".
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