viii., p. 460.]

[Footnote 1247: _Month. Not._, vol. xlv., p. 93.]

[Footnote 1248: _Observatory_, vol. viii., p. 4.]

[Footnote 1249: Denning, _Month. Not._, vol. xxxviii., p. 114.]

[Footnote 1250: _Comptes Rendus_, t. cix., p. 344.]

[Footnote 1251: _Month. Not._, vol. lix., p. 140.]

[Footnote 1252: _Bull. de l'Acad. St. Petersb._, t. xii., p. 95.]

[Footnote 1253: _Publ. Astr. Pac. Soc._, vol. iii., p. 114.]

[Footnote 1254: _Month. Not._, vol. lii., p. 341.]

[Footnote 1255: _Astr. Nach._, No. 1,488.]

[Footnote 1256: _Annuaire_, Paris, 1883, p. 185.]

[Footnote 1257: _Phil. Trans._, vol. clviii., p. 556.]

[Footnote 1258: Hasselberg, _Mem. de l'Ac. Imp. de St. Petersbourg_, t.
xxviii. (7th ser.), No. 2, p. 66.]

[Footnote 1259: Scheiner, _Die Spectralanalyse der Gestirne_, p. 234.
Kayser (_Astr. and Astroph._, vol. xiii., p. 368) refers the anomalies
of the carbon-spectrum in comets wholly to instrumental sources.]

[Footnote 1260: Dewar, _Proc. Roy. Inst._, vol. xi., p. 541.]

[Footnote 1261: _Proc. R. Soc._, vol. xv., p. 5; _Month. Not._, vol.
xxvii., p. 288.]

[Footnote 1262: Keeler, _Astr. and Astrophysics_, vol. xi., p. 929;
Vogel, _Astr. Nach._, No. 3,142.]

[Footnote 1263: _Proc. Roy. Soc._, vol. xxiii., p. 154.]

[Footnote 1264: Hasselberg, _loc. cit._, p. 58.]

[Footnote 1265: _Ueber die Natur der Cometen_, p. 112.]

[Footnote 1266: Hasselberg, _loc. cit._, p. 38.]

CHAPTER XI

_RECENT COMETS_ (_continued_)

The mystery of comets' tails had been to some extent penetrated; so far,
at least, that, by making certain assumptions strongly recommended by
the facts of the case, their forms can be, with very approximate
precision, calculated beforehand. We have, then, the assurance that
these extraordinary appendages are composed of no ethereal or
supersensual stuff, but of matter such as we know it, and subject to the
ordinary laws of motion, though in a state of extreme tenuity.

Olbers, as already stated, originated in 1812 the view that the tails of
comets are made up of particles subject to a force of electrical
repulsion proceeding from the sun. It was developed and enforced by
Bessel's discussion of the appearances presented by Halley's comet in
1835. He, moreover, provided a formula for computing the movement of a
particle under the influence of a repulsive force of any given
intensity, and thus laid firmly the fou