h led to his immortal discovery. He had a list of
seventy stars to observe, fifty of which he observed pretty regularly. It
may seem odd that he did not set up this new instrument at Oxford, but we
find from an old memorandum that his professorship was not bringing him in
quite L140 a year, and probably he was glad to accept his aunt's
hospitality for reasons of economy. By watching these different stars he
gradually got a clear conception of the laws of aberration. The real
solution of the problem, according to a well-authenticated account,
occurred to him almost accidentally. We all know the story of the apple
falling and setting Newton to think about the causes of gravitation. It
was a similarly trivial circumstance which suggested to Bradley the
explanation which he had been seeking for two or three years in vain. In
his own words, "at last, when he despaired of being able to account for
the phenomena which he had observed, a satisfactory explanation of them
occurred to him all at once when he was not in search of it." He
accompanied a pleasure party in a sail upon the river Thames. The boat in
which they were was provided with a mast which had a vane at the top of
it. It blew a moderate wind, and the party sailed up and down the river
for a considerable time. Dr. Bradley remarked that every time the boat put
about the vane at the top of the boat's mast shifted a little, as if there
had been a slight change in the direction of the wind. He observed this
three or four times without speaking; at last he mentioned it to the
sailors, and expressed his surprise that the wind should shift so
regularly every time they put about. The sailors told him that the wind
had not shifted, but that the apparent change was owing to the change in
the direction of the boat, and assured him that the same thing invariably
happened in all cases. This accidental observation led him to conclude
that the phenomenon which had puzzled him so much was owing to the
combined motion of light and of the earth. To explain exactly what is
meant we must again have recourse to a diagram; and we may also make use
of an illustration which has become classical.

[Illustration: FIG. 5.]

[Sidenote: Analogy of rain.]

If rain is falling vertically, as represented by the direction A B; and if
a pedestrian is walking horizontally in the direction C D, the rain will
appear to him to be coming in an inclined direction, E F, and he will find
it better to tilt