into the Julian
calendar, calculated exactly how much of a correction was needed in
order to restore the year to its proper place, and suggested the
method by which future errors of this kind could be avoided. His
ideas were too far beyond his century to be applied practically, but
they were not to be without their effect, and it is said that they
formed the basis of the subsequent correction of the calendar in the
time of Pope Gregory XIII., about three centuries later.

It is rather surprising to find how much besides the theory of
lenses Friar Bacon had succeeded in finding out in the department of
optics. He taught, for instance, the principle of the aberration of
light, and, still more marvelous to consider, taught that light did
not travel instantaneously, but had a definite rate of motion,
though this was extremely rapid. It is rather difficult to
understand how he reached this conclusion, since light travels so
fast that, as far as regards any observation that can be made upon
earth, the diffusion is practically instantaneous. It was not for
over three centuries later that Roemer, the German astronomer,
demonstrated the motion of light and its rate by his observations
upon the moons of Jupiter at different phases of the earth's orbit,
which showed that the light of these moons took a definite and quite
appreciable time to reach the earth after their eclipse by the
planet was over.

{324}

Albertus Magnus's other great pupil besides Roger Bacon was St.
Thomas Aquinas. If any suspicion were still left that Thomas did not
appreciate just what the significance of his teachings in physics
was, when he announced that neither matter nor force could ever be
reduced to nothingness, it would surely be removed by the
consideration that he had been for many years in intimate relations
with Albert, and that he had probably also been close to Roger
Bacon. In association with such men as these, he was not likely to
stumble upon truths unawares, even though they might concern
physical science. St. Thomas himself has left three treatises on
chemical subjects, and it is said that the first occurrence of the
word amalgam can be traced to one of these treatises. Everybody was
as much interested then, as we are at the present time, in the
transformation of metals and mercury with its silvery sheen; its
facility to enter into metallic combinations of all ki