re identical.
The inference is a correct one, but it is in no sense an obvious one.
Long and patient observation established the remarkable law that one of
these bodies was never seen until the other had disappeared. Hence it
was inferred that the phenomena, both at sunrise and at sunset, were due
to the same body, which oscillated to and fro about the sun.

We can easily imagine that the announcement of the identity of these two
objects was one which would have to be carefully tested before it could
be accepted. How are the tests to be applied in a case of this kind?
There can hardly be a doubt that the most complete and convincing
demonstration of scientific truth is found in the fulfilment of
prediction. When Mercury had been observed for years, a certain
regularity in the recurrence of its visibility was noticed. Once a
periodicity had been fully established, prediction became possible. The
time when Mercury would be seen after sunset, the time when it would be
seen before sunrise, could be foretold with accuracy! When it was found
that these predictions were obeyed to the letter--that the planet was
always seen when looked for in accordance with the predictions--it was
impossible to refuse assent to the hypothesis on which these predictions
were based. Underlying that hypothesis was the assumption that all the
various appearances arose from the oscillations of a single body, and
hence the discovery of Mercury was established on a basis as firm as the
discovery of Jupiter or of Venus.

In the latitudes of the British Islands it is generally possible to see
Mercury some time during the course of the year. It is not practicable
to lay down, within reasonable limits, any general rule for finding the
dates at which the search should be made; but the student who is
determined to see the planet will generally succeed with a little
patience. He must first consult an almanac which gives the positions of
the body, and select an occasion when Mercury is stated to be an evening
or a morning star. Such an occasion during the spring months is
especially suitable, as the elevation of Mercury above the horizon is
usually greater then than at other seasons; and in the evening twilight,
about three-quarters of an hour after sunset, a view of this shy but
beautiful object will reward the observer's attention.

To those astronomers who are provided with equatorial telescopes such
instructions are unnecessary. To enjoy a telescopic v