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588 MEDIEVAL ENGLAND
heavenly spheres, the earth, and all the substances and creatures
on it. This 'cosmogony of light* was of Neoplatonic origin. Its
importance in the history of science is, first, that it convinced
Grosseteste himself that optics was the fundamental physical
science; and secondly, because optics could not be studied
without mathematics, that Grosseteste's influence committed a
growing body ofnatural philosophers, both in Oxford and on
the Continent, to the use of mathematical theories, not
only in
optics but also in all possible branches of science.
Grosseteste's own contributions to
optics, apart from the
study of the rainbow, consisted of a partiallycorrect explana^
tion ofthe spherical lens and the suggestion that lenses could be
used to aid weak sight, an unsuccessful attempt to formulate
the law of refraction, and a most suggestive theory that
light
propagates itselfin a series ofpulses or waves. Other contribux
tions were made by Grosseteste's followers. Roger Bacon,
writing about 1266-7, developed his theory ofpropagation in
the
theory known as the
'multiplication ofspecies', designed to
explain action at a distance, whether by light, heat, magnetism,
or
gravity, extended his work on the rainbow and tried to ex'
plain the halo, gave a systematic classification of convex and
concave lenses
(see Fig. 107) and discussed their use as aids to
sight, and used his knowledge both of optics and of anatomy to
try, with partial success, to understand the formation of an
image in the eye. Bacon also discussed the reflecting properties
of surfaces produced by rotating various conic sections about
their axes, stimulated perhaps by the man whom, next to
Grosseteste, he most admired, Petrus Peregrinus de Maricourt,
a Frenchman who made a fundamental experimental study of
the elementary properties of magnets; Bacon described expert
ments which heprobably made himselfwith a floating magnet.
A follower of both Grosseteste and Bacon, the unknown
author ofthe Summa Philosophiae formerly attributed to Grosse^
teste himself, seems to have been the first writer to point out that
the colours produced by passing sunlight through a prism were
refracted through different angles. The whole Summa is an
excellent review of science about 1270, ranging
from astnv