Whether the stars have their light from the sun or in themselves.
Some say that they shine of themselves, alledging that if Venus
nd Mercury had not a light of their own, when they come between
ur eye and the sun they would darken so much of the sun as they
ould cover from our eye. But this is false, for it is proved that
dark object against a luminous body is enveloped and entirely
oncealed by the lateral rays of the rest of that luminous body
nd so remains invisible. As may be seen when the sun is seen
hrough the boughs of trees bare of their leaves, at some distance
he branches do not conceal any portion of the sun from our eye.
he same thing happens with the above mentioned planets which,
hough they have no light of their own, do not--as has been said--
onceal any part of the sun from our eye
[18].

SECOND ARGUMENT.

Some say that the stars appear most brilliant at night in proportion
as they are higher up; and that if they had no light of their own,
the shadow of the earth which comes between them and the sun, would
darken them, since they would not face nor be faced by the solar
body. But those persons have not considered that the conical shadow
of the earth cannot reach many of the stars; and even as to those it
does reach, the cone is so much diminished that it covers very
little of the star's mass, and all the rest is illuminated by the
sun.

Footnote: From this and other remarks (see No. 902) it is clear
hat Leonardo was familiar with the phenomena of Irradiation.]