lution; MM. Laugier and Mauvais of Paris considered the
true term to be 35;[293] Clausen looked for its return at the end of
between six and seven. A recent discussion[294] by Professor Kreutz of
all the available data gives a probable period of 512 years for this
body, and precludes its hypothetical identity with the comet of 1668,
known as the "Spina" of Cassini.

It may now be asked, what were the conclusions regarding the nature of
comets drawn by astronomers from the considerable amount of novel
experience accumulated during the first half of this century? The first
and best assured was that the matter composing them is in a state of
extreme tenuity. Numerous and trustworthy observations showed that the
feeblest rays of light might traverse some hundreds of thousands of
miles of their substance, even where it was apparently most condensed,
without being perceptibly weakened. Nay, instances were recorded in
which stars were said to have gained in brightness from the
process![295] On the 24th of June, 1825, Olbers[296] saw the comet then
visible all but obliterated by the central passage of a star too small
to be distinguished with the naked eye, its own light remaining wholly
unchanged. A similar effect was noted December 1, 1811, when the great
comet of that year approached so close to Altair, the _lucida_ of the
Eagle, that the star seemed to be transformed into the nucleus of the
comet.[297] Even the central blaze of Halley's comet in 1835 was
powerless to impede the passage of stellar rays. Struve[298] observed at
Dorpat, on September 17, an all but central occultation; Glaisher[299]
one (so far as he could ascertain) absolutely so eight days later at
Cambridge. In neither case was there any appreciable diminution of the
star's light. Again, on the 11th of October, 1847, Mr. Dawes,[300] an
exceptionally keen observer, distinctly saw a star of the tenth
magnitude through the exact centre of a comet discovered on the first of
that month by Maria Mitchell of Nantucket.

Examples, on the other hand, are not wanting of the diminution of
stellar light under similar circumstances;[301] and we meet two alleged
instances of the vanishing of a star behind a comet. Wartmann of Geneva
observed the first, November 28, 1828;[302] but his instrument was
defective, and the eclipsing body, Encke's comet, has shown itself
otherwise perfectly translucent. The second case of occultation occurred
September 13, 1890, when an elevent