y years ago, supposed to be
due in some way to the action of the planets. But, for reasons which it
would be tedious to go into at present, we may fairly regard this
hypothesis as being completely disproved. There can, I conclude, be
little doubt that the eleven-year cycle of change in the solar spots is
due to a cycle going on in the sun itself. Such being the case, the
corresponding change in the earth's magnetism must be due to the same
cause.

We may, therefore, regard it as a fact sufficiently established to
merit further investigation that there does emanate from the sun, in an
irregular way, some agency adequate to produce a measurable effect on
the magnetic needle. We must regard it as a singular fact that no
observations yet made give us the slightest indication as to what this
emanation is. The possibility of defining it is suggested by the
discovery within the past few years, that under certain conditions,
heated matter sends forth entities known as Rontgen rays, Becquerel
corpuscles and electrons. I cannot speak authoritatively on this
subject, but, so far as I am aware, no direct evidence has yet been
gathered showing that any of these entities reach us from the sun. We
must regard the search for the unknown agency so fully proved as among
the most important tasks of the astronomical physicist of the present
time. From what we know of the history of scientific discovery, it
seems highly probable that, in the course of his search, he will,
before he finds the object he is aiming at, discover many other things
of equal or greater importance of which he had, at the outset, no
conception.

The main point I desire to bring out in this review is the tendency
which it shows towards unification in physical research. Heretofore
differentiation--the subdivision of workers into a continually
increasing number of groups of specialists--has been the rule. Now we
see a coming together of what, at first sight, seem the most widely
separated spheres of activity. What two branches could be more widely
separated than that of stellar statistics, embracing the whole universe
within its scope, and the study of these newly discovered emanations,
the product of our laboratories, which seem to show the existence of
corpuscles smaller than the atoms of matter? And yet, the phenomena
which we have reviewed, especially the relation of terrestrial
magnetism to the solar activity, and the formation of nebulous masses
around the new