During many
centuries the theory favoured by the Church had been, as we have
seen, that the earth was surrounded by hollow spheres, concentric
and transparent, forming a number of glassy strata incasing one
another "like the different coatings of an onion," and that each
of these in its movement about the earth carries one or more of the
heavenly bodies. Some maintained that these spheres were crystal;
but Lactantius, and with him various fathers of the Church, spoke
of the heavenly vault as made of ice. Now, the admission that
comets could move beyond the moon was fatal to this theory, for it
sent them crashing through these spheres of ice or crystal, and
therefore through the whole sacred fabric of the Ptolemaic theory.[202]
Here we may pause for a moment to note one of the chief differences
between scientific and theological reasoning considered in
themselves. Kepler's main reasoning as to the existence of a law
for cometary movement was right; but his secondary reasoning, that
comets move nearly in straight lines, was wrong.
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