And Thirdly, That it is a very _short-vibrating motion_, I think the instances drawn from the shining of Diamonds will also make probable. For a Diamond being the hardest body we yet know in the World, and consequently the least apt to yield or bend, must consequently also have its _vibrations_ exceeding short. And these, I think, are the three principal proprieties of a motion, requisite to produce the effect call’d Light in the Object. The next thing we are to consider, is the way or manner of the _trajection_ of this motion through the interpos’d pellucid body to the eye: And here it will be easily granted, 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 _vibration_ 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