le oscillation, the period
of one being 427 days and the other about a year, the mean amplitude of
each being O".14. In other words, the spot in the arctic regions,
directly in the prolongation of the earth's axis of rotation, is not
absolutely fixed; the end of the imaginary axis moves about in a
complicated manner, but always keeping within a few yards of its average
position. This remarkable discovery is not only of value as introducing
a new refinement in many astronomical researches depending on an
accurate knowledge of the latitude, but theoretical investigations show
that the periods of this variation are incompatible with the assumption
that the earth is an absolutely rigid body. Though this assumption has
in other ways been found to be untenable, the confirmation of this view
by the discovery of Dr. Chandler is of great importance.

CHAPTER XXV.

THE ABERRATION OF LIGHT.

The Real and Apparent Movements of the Stars--How they can be
Discriminated--Aberration produces Effects dependent on the
Position of the Stars--The Pole of the Ecliptic--Aberration makes
Stars seem to Move in a Circle, an Ellipse, or a Straight Line
according to Position--All the Ellipses have Equal Major Axes--How
is this Movement to be Explained?--How to be Distinguished from
Annual Parallax--The Apex of the Earth's Way--How this is to be
Explained by the Velocity of Light--How the Scale of the Solar
System can be Measured by the Aberration of Light.

We have in this chapter to narrate a discovery of a recondite character,
which illustrates in a forcible manner some of the fundamental truths of
Astronomy. Our discussion of it will naturally be divided into two
parts. In the first part we must describe the nature of the phenomenon,
and then we must give the extremely elegant explanation afforded by the
properties of light. The telescopic discovery of aberration, as well as
its explanation, are both due to the illustrious Bradley.

The expression _fixed_ star, so often used in astronomy, is to be
received in a very qualified sense. The stars are, no doubt, well fixed
in their places, so far as coarse observation is concerned. The
lineaments of the constellations remain unchanged for centuries, and, in
contrast with the ceaseless movements of the planets, the stars are not
inappropriately called fixed. We have, however, had more than one
occasion to show throughout the course of this wo