colour of the sun, and when it is
less hot it has the colour of fire.

37.

It is proved that the sun is by nature hot and not cold, as has already
been stated. If rays of fire play on a concave mirror when it is cold,
the rays refracted by the mirror will be hotter than {156} the fire.
The rays emitted from a sphere of glass filled with cold water, which
are reflected from a fire, will be warmer than the fire. It follows
from these two experiments that the heat of the rays reflected by the
mirror or the sphere of cold water are hot by virtue, and not because
the mirror or the sphere is hot; and in this case it occurs that the
sun, passing through these bodies, heats them by its virtue. And owing
to this they have inferred that the sun is not hot,--which by the
aforesaid experiments has been proved to be exceedingly hot, by the
experiment of the mirror and the sphere, which are cold in themselves,
and reflect the hot rays of the fire and render them hotter, because
the first cause is hot; and the same thing occurs as regards the sun,
which, being hot in itself, and passing through these cold mirrors,
refracts great heat. It is not the light of the sun which gives
warmth, but its natural heat.

[Sidenote: Rays of the Sun]

38.

The rays of the sun pass through the cold region of the air without any
change being effected in their nature, they pass through glasses full
of cold water without suffering change; through whatever transparent
spot they pass, it is as though they passed through so much air.

[Sidenote: Light of the Stars]

39.

Some writers allege that the stars shine of {157} themselves, saying
that if Venus and Mercury did not shine of themselves, when their light
comes between them and the sun they would darken as much of the sun as
they could hide from our eye; this is false, because it is proved that
a dark body placed against a luminous body is enveloped and altogether
covered by the lateral rays of the remaining part of that body, and
thus remains invisible; as may be proved when the sun is seen through
the boughs of a leafless tree at a long distance, the boughs do not
hide any portion of the sun from our eyes. The same thing occurs with
the above-mentioned planets, which, though they have no light in
themselves, do not, as has been said, hide any portion of the sun from
our eyes.

Second proof. They say that the stars shine most brightly at night in
proportion as they are high;