by observation that the
amount of aberration depends upon the distance from the apex. A star
which happened to lie on the ecliptic will not be at all deranged by
aberration from its mean place when it happens that the apex coincides
with the star. All the stars 10 deg. from the apex will be displaced each by
the same amount, and all directly in towards the apex. A star 20 deg. from
the apex will undergo a larger degree of displacement, though still in
the same direction, exactly towards the apex; and all stars at the same
distance will be displaced by the same amount. Proceeding thus from the
apex, we come to stars at a distance of 90 deg. therefrom. Here the amount
of displacement will be a maximum. Each one will be about twenty seconds
from its average place; but in every case the imperative law will be
obeyed, that the displacement of the star from its mean place lies
towards the apex of the earth's way. We have thus given two distinct
descriptions of the phenomenon of aberration. In the first we find it
convenient to speak of a star as describing a minute circular path; in
the other we have regarded aberration as merely amounting to a
derangement of the star from its mean place in accordance with specified
laws. These descriptions are not inconsistent: they are, in fact,
geometrically equivalent; but the latter is rather the more perfect,
inasmuch as it assigns completely the direction and extent of the
derangement caused by aberration in any particular star at any
particular moment.

The question has now been narrowed to a very definite form. What is it
which makes each star seem to close in towards the point towards which
the earth is travelling? The answer will be found when we make a minute
enquiry into the circumstances in which we view a star in the telescope.

The beam of rays from a star falls on the object-glass of a telescope;
those rays are parallel, and after they pass through the object-glass
they converge to a focus near the eye end of the instrument. Let us
first suppose that the telescope is at rest; then if the telescope be
pointed directly towards the star, the rays will converge to a point at
the centre of the field of view where a pair of cross wires are placed,
whose intersection defines the axis of the telescope. The case will,
however, be altered if the telescope be moved after the light has passed
through the objective; the rays of light in the interior of the tube
will pursue a direct path,