long as
there is room for refusing to admit an important theory advanced by a
student of science, it is natural that other students of science should
refuse to do so; for in admitting the new theory they are awarding the
palm to a rival. In strict principle, of course, this consideration
ought to have no influence whatever; as a matter of fact, however, men
of science, being always men and not necessarily strengthened by
scientific labours against the faults of humanity, the consideration has
and must always have influence. Therefore, when the fellow-writers and
rivals of Newton or of his followers gave in their adhesion to the
Newtonian theory; when in our own time--but let us leave our own time
alone, in this respect--when, speaking generally, a novel doctrine, or
some new generalisation, or some great and startling discovery, is
admitted by rival students of the branch of astronomy to which it
belongs, the probability is great that the weight of evidence has been
found altogether overwhelming.

Let us now, however, turn to cases in which, while many observations
seem to point to some result, it has appeared that, after all, those
observations must have been illusory.

A striking instance in point is found in the perplexing history of the
supposed satellite of Venus.

On January 25, 1672, the celebrated astronomer, J.D. Cassini saw a
crescent shaped and posited like Venus, but smaller, on the western side
of the planet. More than fourteen years later, he saw a crescent east of
the planet. The object continued visible in the latter case for half an
hour, when the approach of daylight obliterated the planet and this
phantom moon from view. The apparent distance of the moon from Venus was
in both cases small, viz., only one diameter of the planet in the former
case, and only three-fifths of that diameter in the latter.

Next, on October 23, 1740, old style, the optician Short, who had had
considerable experience in observation, saw a small star perfectly
defined but less luminous than Venus, at a distance from the planet
equal to about one-third of the apparent diameter of our moon. This is a
long distance, and would correspond to a distance from Venus certainly
not less than the moon's distance from the earth. Short was aware of the
risk of optical illusion in such matters, and therefore observed Venus
with a second telescope; he also used four eye-pieces of different
magnifying power. He says that Venus was very di