This ethereal medium, according to our author, is exceedingly more rare and more elastic than air. It pervades all bodies, and is expanded through all the heavens. It is much rarer within the dense bodies of the sun, stars, planets, and comets, than in the celestial spaces between them, and also more rare within glass, water, &c. than in the free and open spaces void of air and other grosser bodies. In passing out of glass, water, &c. and other dense bodies into empty space, it grows denser and denser by degrees, and this gradual condensation extends to some distance from the bodies. Owing to its great elasticity, and, consequently, its efforts to spread in all directions, it presses against itself, and, consequently, against the solid particles of bodies, so as to make them continually approach to one another, the body being impelled from the denser parts of the medium towards the rarer with all that power which we call gravity. In employing this medium to explain the nature of light, Newton does not suppose, with Descartes, Hooke, Huygens, and others, that light is nothing more than the impression of those undulations on the retina. He regards light as a peculiar substance, composed of heterogeneous particles thrown off with great velocity, and in all directions, from luminous bodies; and he supposes that these particles while passing through the ether, excite in it vibrations or pulses which accelerate or retard the particles of light, and thus throw them into their alternate fits of easy reflection and transmission. Hence, if a ray of light falls upon a transparent body, in which the ether consists of strata of variable density, the particles of light acted upon by the vibrations which they create will be urged with an accelerated velocity in entering the body, while their velocity will be retarded in quitting it. In this manner he conceives the phenomena of refraction to be produced, and he shows how in such a case the refraction would be regulated by the law of the sines.