171. When a ray of light passes out of one medium[38] into another, it
is refracted, or turned out of it’s first course, more or less, as it
falls more or less obliquely on the refracting surface which divides the
two mediums. This may be proved by several experiments; of which we
shall only give three for example’s sake. 1. In a bason _FGH_ put a
piece of money as _DB_, and then retire from it as to _A_, till the edge
of the bason at _E_ just hides the money from your sight: then, keeping
your head steady, let another person fill the bason gently with water.
As he fills it, you will see more and more of the piece _DB_; which will
be all in view when the bason is full, and appear as if lifted up to
_C_. For, the ray _AEB_, which was straight whilst the bason was empty,
is now bent at the surface of the water in _E_, and turned out of it’s
rectilineal course into the direction _ED_. Or, in other words, the ray
_DEK_, that proceeded in a straight line from the edge _D_ whilst the
bason was empty, and went above the eye at _A_, is now bent at _E_; and
instead of going on in the rectilineal direction _DEK_, goes in the
angled direction _DEA_, and by entering the eye at _A_ renders the
object _DB_ visible. Or, 2dly, place the bason where the Sun shines
obliquely, and observe where the shadow of the rim _E_ falls on the
bottom, as at _B_: then fill it with water, and the shadow will fall at
_D_; which proves, that the rays of light, falling obliquely on the
surface of the water, are refracted, or bent downwards into it.
172. The less obliquely the rays of light fall upon the surface of any
medium, the less they are refracted; and if they fall perpendicularly
thereon, they are not refracted at all. For, in the last experiment, the
higher the Sun rises, the less will be the difference between the places
where the edge of the shadow falls, in the empty and full bason. And,
3dly, if a stick be laid over the bason, and the Sun’s rays be reflected
perpendicularly into it from a looking-glass, the shadow of the stick
will fall upon the same place of the bottom, whether the bason be full
or empty.
173. The denser that any medium is, the more is light refracted in
passing through it.
[Sidenote: The Atmosphere.