CHAPTER IV

ON THE REFRACTION OF THE AIR


We have shown how the movement which constitutes light spreads by
spherical waves in any homogeneous matter. And it is evident that when
the matter is not homogeneous, but of such a constitution that the
movement is communicated in it more rapidly toward one side than
toward another, these waves cannot be spherical: but that they must
acquire their figure according to the different distances over which
the successive movement passes in equal times.

It is thus that we shall in the first place explain the refractions
which occur in the air, which extends from here to the clouds and
beyond. The effects of which refractions are very remarkable; for by
them we often see objects which the rotundity of the Earth ought
otherwise to hide; such as Islands, and the tops of mountains when one
is at sea. Because also of them the Sun and the Moon appear as risen
before in fact they have, and appear to set later: so that at times
the Moon has been seen eclipsed while the Sun appeared still above the
horizon. And so also the heights of the Sun and of the Moon, and those
of all the Stars always appear a little greater than they are in
reality, because of these same refractions, as Astronomers know. But
there is one experiment which renders this refraction very evident;
which is that of fixing a telescope on some spot so that it views an
object, such as a steeple or a house, at a distance of half a league
or more. If then you look through it at different hours of the day,
leaving it always fixed in the same way, you will see that the same
spots of the object will not always appear at the middle of the