First, That it must be a body _susceptible_ and _impartible_ of this motion
that will deserve the name of a Transparent. And next, that the parts of
such a body must be _Homogeneous_, or of the same kind. Thirdly, that the
constitution and motion of the parts must be such, that the appulse of the
luminous body may be communicated or propagated through it to the greatest
imaginable distance in the least imaginable time, though I see no reason to
affirm, that it must be in an instant: For I know not any one Experiment or
observation that does prove it. And, whereas it may be objected, That we
see the Sun risen at the very instant when it is above the sensible
Horizon, and that we see a Star hidden by the body of the Moon at the same
instant, when the Star, the Moon, and our Eye are all in the same line; and
the like Observations, or rather suppositions, may be urg'd. I have this to
answer, That I can as easily deny as they affirm; for I would fain know by
what means any one can be assured any more of the Affirmative, then I of
the Negative. If indeed the propagation were very slow, 'tis possible
something might be discovered by Eclypses of the Moon; but though we should
grant the progress of the light from the Earth to the Moon, and from the
Moon back to the Earth again to be full two Minutes in performing, I know
not any possible means to discover it; nay, there may be some instances
perhaps of Horizontal Eclypses that may seem very much to favour this
supposition of the slower progression of Light then most imagine. And the
like may be said of the Eclypses of the Sun, &c. But of this only by the
by. Fourthly, That the motion is propagated every way through an
_Homogeneous medium_ by _direct_ or _straight_ 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