Hence it will easily be imagined that our knowledge of the corona, the
part of the sun which we can still only study on occasions of a total
solar eclipse, advances but slowly. During the last twenty years there has
been altogether scarcely half-an-hour available for this research, though
it may fairly be said that the very best possible use has been made of
that half-hour. And, what is of importance for our immediate purpose, it
has gradually been established by comparing the photographs of one eclipse
with those of another, that the corona itself undergoes distinct changes
in form in the same period which governs the changes of sun-spots. When
there are many sun-spots the corona spreads out in all directions from the
edge of the sun's disc; when there are few sun-spots the corona extends
very much further in the direction of the sun's equator, so that at
sun-spot minimum there is an appearance of two huge wings. Although the
evidence is necessarily collected in a scrappy manner, by this time there
is sufficient to remove this relationship out of the region of mere
suspicion, and to give it a well-established place in our knowledge of the
sun's surroundings.