lines extended every way
like Rays from the center of a Sphere. Fifthly, in an _Homogeneous medium_
this motion is propagated every way with _equal velocity_, whence
necessarily every _pulse_ or _vitration_ of the luminous body will generate
a Sphere, which will continually increase, and grow bigger, just after the
same manner (though indefinitely swifter) as the waves or rings on the
surface of the water do swell into bigger and bigger circles about a point
of it, where, by the sinking of a Stone the motion was begun, whence it
necessarily follows, that all the parts of these Spheres undulated through
an _Homogeneous medium_ cut the Rays at right angles.

But because all transparent _mediums_ are not _Homogeneous_ to one another,
therefore we will next examine how this pulse or motion will be propagated
through differingly transparent _mediums_. And here, according to the most
acute and excellent Philosopher _Des Cartes_, I suppose the sign of the
angle of inclination in the first _medium_ to be to the sign of refraction
in the second, As the density of the first, to the density of the second.
By density, I mean not the density in respect of gravity (with which the
refractions or transparency of _mediums_ hold no proportion) but in respect
onely to the _trajection_ of the Rays of light, in which respect they only
differ in this; that the one propagates the pulse more easily and weakly,
the other more slowly, but more strongly. But as for the pulses themselves,
they will by the refraction acquire another propriety, which we shall now
endeavour to explicate.

We will suppose therefore in the first Figure ACFD to be a physical Ray, or
ABC and DEF to be two Mathematical Rays, _trajected_ from a very remote
point of a luminous body through an _Homogeneous_ transparent _medium_ LLL,
and DA, EB, FC, to be small portions of the orbicular impulses which must
therefore cut the Rays at right angles; these Rays meeting with the plain
surface NO of a _medium_ that yields an easier _transitus_ to the
propagation of light, and falling _obliquely_ on it, they will in the
_medium_ MMM be refracted towards the perpendicular of the surface. And
because this _medium_ is more easily _trajected_ then the former by a
third, therefore the point C of the orbicular pulse FC will be mov'd to H
four spaces in the same time that F the other end of it is mov'd to G three
spaces, therefore the whole refracted pulse GH shall be _oblique_ to the
refra