ntific knowledge of that day had not bulked so large as
to exclude the possibility that one man might master it all. So we find
a Galileo, for example, making revolutionary discoveries in astronomy,
and performing fundamental experiments in various fields of physics.
Galileo's great contemporary, Kepler, was almost equally versatile,
though his astronomical studies were of 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 law, as Descartes expressed it, states that the sine
of the angle of incidence bears a fixed ratio to the sine of the angle
of refraction for any given medium. Here, then, was another illustration
of the fact that almost infinitely varied phenomena may be brought
within the scope of a simple law. Once the law had been expressed, it