ies in observing very exactly the beginning and end of the
transit. There was an appearance called the "Black Drop," which had caused
trouble on previous occasions; an appearance as though the round black
spot which can be seen when Venus has advanced some distance over the
sun's disc was reluctant to make the entry and clung to the edge or "limb"
of the sun as it is called, somewhat as a drop of ink clings to a pen
which is slowly withdrawn from an inkpot. Similarly, at the end of the
transit or egress, instead of approaching the limb steadily the planet
seems at the last moment to burst out towards it, rendering the estimation
of the exact moment when the transit is over extremely doubtful.

[Sidenote: Failure.]

These difficulties, as already stated, were known to exist; but there is a
long interval between transits of Venus, or rather between every pair of
such transits. After those of 1874 and 1882 there will be no more until
2004 and 2012, so that we shall never see another; similarly, before that
pair of the last century, there had not been any such occasion since 1761
and 1769, and no one was alive who remembered at first hand the trouble
which was known to exist. It was proposed to obviate the anticipated
difficulties by careful practice beforehand; models were prepared to
resemble as nearly as possible the expected appearances, and the times
recorded by different observers were compared with the true time, which
could, in this case of a model, be determined. In this way it was hoped
that the habit of each observer, his "personal equation" as it is called,
could be determined beforehand, and allowed for as a correction when he
came to observe the actual transit. The result, however, was a great
disappointment. The actual appearances were found to be totally different
in character from those shown by the model; chiefly, perhaps, because it
had been impossible to imitate with a model the effect of the atmosphere
which surrounds the planet Venus. Observers trained beforehand, using
similar instruments, and standing within a few feet of each other, were
expected, after making due allowance for personal equation, to give the
same instant for contact; but their observations when made were found to
differ by nearly a minute of time, and after an exhaustive review of the
whole material it was felt that all hope of determining accurately the
sun's distance by this method must be given up. The following table will
show h