such pre-eminent importance that his other investigations sink
into relative insignificance. Yet he performed some notable
experiments in at least one department of physics. These
experiments had to do with the refraction of light, a subject
which Kepler was led to investigate, in part at least, through
his interest in the telescope.

We have seen that Ptolemy in the Alexandrian time, and Alhazen,
the Arab, made studies of refraction. Kepler repeated their
experiments, and, striving as always to generalize his
observations, he attempted to find the law that governed the
observed change of direction which a ray of light assumes in
passing from one medium to another. Kepler measured the angle of
refraction by means of a simple yet ingenious trough-like
apparatus which enabled him to compare readily the direct and
refracted rays. He discovered that when a ray of light passes
through a glass plate, if it strikes the farther surface of the
glass at an angle greater than 45 degrees it will be totally
refracted instead of passing through into the air. He could not
well fail to know that different mediums refract light
differently, and that for the same medium the amount of light
valies with the change in the angle of incidence. He was not
able, however, to generalize his observations as he desired, and
to the last the law that governs refraction escaped him. It
remained for Willebrord Snell, a Dutchman, about the year 1621,
to discover the law in question, and for Descartes, a little
later, to formulate it. Descartes, indeed, has sometimes been
supposed to be the discoverer of the law. There is reason to
believe that he based his generalizations on the experiment of
Snell, though he did not openly acknowledge his indebtedness. The