1. Theory and Experiment. These precursors worked
primarily in the area of mechanics, concentrating on
logical and mathematical analyses that led to somewhat
abstract formulations, only much later put to experi-
mental test. They never reached the stage of active
interchange between theory and experiment that char-
acterizes twentieth-century science, and that could
only be begun in earnest with the mechanical investi-
gations of Galileo and Newton. In another area of
study, however, a beginning was made even in this type
of methodology; the area, predictably enough, was
optics, which from antiquity had been emerging, along
with mechanics, as an independent branch of physics.
The reasons for the privileged position enjoyed by
optics in the late thirteenth and early fourteenth cen-
turies are many. One was the eminence it earlier had
come to enjoy among the Greeks and the Arabs. An-
other was its easy assimilation within the theological
context of “Let there be light” (Fiat lux) and the philo-
sophical context of the “metaphysics of light” already
alluded to. Yet other reasons can be traced in the
striking appearances of spectra, rainbows, halos, and
other optical phenomena in the upper atmosphere, in
the perplexity aroused by optical delusions or by an
awareness of their possibility, and above all in the
applicability of a simple geometry toward the solution
of optical problems.
Whatever the reasons, the fact is that considerable
progress had already been made in both catoptrics, the
study of reflected light, and dioptrics, the study of
refraction. In the former, the works of Euclid, Ptolemy,
and Alhazen (d. 1038) had shown that the angles of
incidence and reflection from plane surfaces are equal;
they also explained how images are formed in plane
mirrors and, in the case of Alhazen, gave exhaustive
and accurate analyses of reflection from spherical and
parabolic mirrors. Similarly in dioptrics Ptolemy and
Alhazen had measured angles of incidence and refrac-
tion, and knew in a qualitative way the difference
between refraction away from, and refraction toward,
the normal, depending on the media through which
the light ray passed. Grosseteste even attempted a
quantitative description of the phenomenon, proposing
that the angle of refraction equals half the angle of
incidence, which is, of course, erroneous. In this way,
however, the stage was gradually set for more substan-
tial advances in optics by Witelo and Dietrich von
Freiberg. Perhaps the most remarkable was Dietrich's
work on the rainbow (De iride), composed shortly after
1304, wherein he explained the production of the bow
through the refraction and reflection of light rays.
Dietrich's treatise is lengthy and shows considerable
expertise in both experimentation and theory, as well
as the ability to relate the two. On the experimental
side Dietrich passed light rays through a wide variety
of prisms and crystalline spheres to study the produc-
tion of spectra. He traced their paths through flasks
filled with water, using opaque surfaces to block out
unwanted rays, and obtained knowledge of angles of
refraction at the various surfaces on which the rays
in which he was interested were incident, as well as
the mechanics of their internal reflection within the
flask. Using such techniques he worked out the first
essentially correct explanation of the formation of the
primary and secondary rainbows (Figures 1 and 2). The
theoretical insight that lay behind this work, and that
had escaped all of his predecessors, was that a globe
of water could be thought of—not as a diminutive
watery cloud, as others viewed it—but as a magnified
raindrop. This, plus the recognition that the bow is
actually the cumulative effect of radiation from many
drops, provided the principles basic to his solution.
Dietrich's experimental genius enabled him to utilize
these principles in a striking way: the first to im-
mobilize the raindrop, in magnified form, in what
would later be called a “laboratory” situation, he was
able to examine leisurely and at length the various
components involved in the rainbow's production.
Dietrich proposed the foregoing methodology as an
application of Aristotle's Posterior Analytics wherein
he identified the causes of the bow and demonstrated
its properties using a process of resolution and com-
position. In attempting to explain the origin and order-
ing of the bow's colors, however, he engaged in a far
more hypothetical type of reasoning, and coupled this
with experiments designed to verify and falsify his
alternative hypotheses. This work, while closer meth-
odologically to that of modern science, was not suc-
cessful. There were errors too in his geometry, and in
some of his measurements; these were corrected in
succeeding centuries, mainly by Descartes and Newton.
Dietrich's contribution, withal, was truly monumental,
and represents the best interplay between theory and
experiment known in the high Middle Ages.
