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586
MEDIEVAL ENGLAND
phenomenon was, according to Grosseteste, to find the condi"
tions necessary and sufficient to produce it. The inquiry began
with a 'resolution' ofthe phenomenon into its elements, and of
this process Bacon gave an excellent example in describing how
he collected instances of colours similar to those seen in the
rainbow, so that the rainbow could be related to the general
phenomenon of spectral
colours. He examined the colours seen
in rainbows, in spray made by mill-wheels and by squirting
water from the mouth, in sunlight passed through a glass flask
full of water or through a glass prism or hexagonal crystal on
to a screen, and in different kinds of iridescent feathers. He
concluded that an essential condition for the production of a
rainbow was the presence of spherical water-drops in the atmo-
sphere; he showed also, by means ofan astrolabe, that the rain/
bow was always seen at an angle ofabout 42 from the incident
light going from the sun to the drops.
The next stage was to find out how these conditions operated
to produce a rainbow, and for this Grosseteste, Bacon, and
their successors used the fruitful device of constructing a theo
retical model. Grosseteste's model supposed that a cloud as a
whole acted as a huge refracting lens, Bacon's that the effect
was produced by the reflection of sunlight from the outer sur
faces of individual raindrops. Neither will stand detailed ex
amination, but, though they did not grasp their faults, both
investigators did test the models they considered by subjecting
consequences deduced from them to experiment. Later contv
nental investigators, Albertus Magnus, Witelo, and Theodoric
of Freiberg, all
directly or indirectly influenced by the work
of Grosseteste and Bacon, continued their work of searching
for an adequate theoretical model; Theodoric, shortly before
1311, finally constructed a successful theory, based on the
fundamental discovery that the sunlight entering each raindrop
was not only refracted, and thus broken up into colours, but
also reflected internally by the concave surface, which returned
the colours to the eye ofthe observer. This same model was to
be used by Descartes and Newton.
In using this process of experimental verification andfalsi-