if we travel to
such favoured spots the whole time during which the sun is totally
eclipsed cannot exceed a few minutes, and hence observations are made
under rather hurried and trying conditions. In these modern days of
photography it is easier to take advantage of these precious moments than
it used to be when there was only the eye and memory of an excited
observer to rely upon. It is perhaps not surprising that some of the
evidence collected on these earlier occasions was conflicting; but
nowadays the observers, generally speaking, direct their energies in the
first place to mounting accurately in position photographic apparatus of
different kinds, each item of it specially designed to settle some
particular problem in the most feasible way; secondly, to rehearsing very
carefully the exact programme of exposures necessary during the critical
few minutes; and finally, to securing these photographs with as few
mistakes as possible when the precious moments actually arrive. Even then
the whole of their efforts are quite likely to be rendered unavailing by a
passing cloud; and bitter is the disappointment when, after travelling
thousands of miles, and spending months in preparation, the whole
enterprise ends in nothing owing to some caprice of the weather.

[Sidenote: Corona follows spots.]

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