the
night of June 29, produced--if any change--an increase of brilliancy in
the object of this spontaneous experiment.[1293] Dr. Meyer, indeed, at
the Geneva Observatory, detected apparent signs of refractive action
upon rays thus transmitted;[1294] but his observations remain isolated,
and were presumably illusory.

The track pursued by this comet gave peculiar advantages for its
observation. Ascending from Auriga through Camelopardus, it stood, July
19, on a line between the Pointers and the Pole, within 8 deg. of the
latter, and thus remained for a lengthened period constantly above the
horizon of northern observers. Its brightness, too, was no transient
blaze, but had a lasting quality which enabled it to be kept steadily in
view during nearly nine months. Visible to the naked eye until the end
of August, the last telescopic observation of it was made February 14,
1882, when its distance from the earth considerably exceeded 300 million
miles. Under these circumstances, the knowledge acquired of its orbit
was of more than usual accuracy, and showed conclusively that the comet
was not a simple return of Bessel's; for this would involve a period of
seventy-four years, whereas Tebbutt's comet cannot revisit the sun until
after the lapse of two and a half millenniums.[1295] Nevertheless, the
twin bodies move so nearly in the same path that an original connection
of some kind is obvious; and the recent example of Biela readily
suggested a conjecture as to what the nature of that connection might
have been. The comets of 1807 and 1881 are, then, regarded with much
probability as fragments of a primitive disrupted body, one following in
the wake of the other at an interval of seventy-four years.

Imperfect photographs were taken of Donati's comet both in England and
America;[1296] but Tebbutt's comet was the first to which the process
was satisfactorily applied. The difficulties to be overcome were very
great. The chemical intensity of cometary light is, to begin with,
extraordinarily small. Janssen estimated it at 1/300000 of
moonlight.[1297] Hence, if the ordinary process by which lunar
photographs are taken had been applied to the comet of 1881, an exposure
of at least _three days_ would have been required in order to get an
impression of the head with about a tenth part of the tail. But by that
time a new method of vastly increased sensitiveness had been rendered
available, by which dry gelatine-plates were substituted