rough the transparent medium, not really, but by
intention, as it were through transparent glass. And in the humour
which is in the pupil of the eye this current which makes the form
visible is completed, because that humour is closed behind like a
mirror which has its glass backed with lead; so that it cannot pass
farther on, but strikes there, after the manner of a ball, and stops;
so that the form which does not appear in the transparent medium,
having reached the disc behind, shines brightly thereon; and this is
the reason why the image appears only in the glass which has lead at
the back.

From this pupil the visual spirit, which is continued from it to the
part of the Brain, the anterior, where the sensitive power is,
suddenly, without loss of time, depicts it as in the clear spring of a
fountain; and thus we see. Wherefore, in order that its vision be
truthful, that is, such as the visible thing is in itself, the medium
through which the form comes to the eye must be without any colour,
and so also the humour of the pupil; otherwise the visible form would
be stained of the colour of the medium and of that of the pupil. And
this is the reason why they who wish to make things appear of a
certain colour in a mirror interpose that colour between the glass and
the lead, the glass being pressed over it.

Plato and other Philosophers said, indeed, that our sight was not
because the visible came into the eye, but because the visual virtue
went out to the visible form. And this opinion is confuted by the
Philosopher in that book of his on Sense and Sensation. Having thus
considered this law of vision, one can easily perceive how, although
the star is always in one way bright, clear, and resplendent, and
receives no change whatever except that of local movement, as is
proved in that book on Heaven and the World, yet from many causes it
may appear dim and obscure; since it may appear thus on account of the
medium, the atmosphere, that changes continually. This medium changes
from light to darkness, according to the presence or absence of the
Sun; and during the presence of the Sun the medium, which is
transparent, is so full of light that it overpowers the star, and
therefore it no longer appears brilliant. This medium also changes
from rare to dense, from dry to moist, because of the vapours of the
Earth which rise continually. The medium, thus changed, changes by its
density the image of the star, which passes through i