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sufficiently attenuated; moreover its density must probably differ in them according to the nature of the substances, since unequal refractions appear to prove that the propagation of light takes place in different media with various velocities. But what ought to be the proportions of the densities for these different substances? How is the luminiferous ether brought to, or kept in the proper state for each? How is it inclosed and contained so as to be incapable of spreading out of them? Moreover, how is this medium, so nonresisting, so rare, so intangible, agitated by the molecules of bodies which appear to us luminous? There are so many characters which it would be necessary to know well, or at least to define well, to have an exact idea of the conditions according to which the undulations are formed and propagated; but hitherto they have never been distinctly established.
At any rate, if a body be conceived to have the faculty of exciting an instantaneous agitation in a point of such a medium, supposed at first equally dense in all its extent, this agitation will be propagated in concentric spherical waves, in the same manner as in air, except that the velocity will be much more considerable. Each molecule of the medium will then be agitated in its turn, and afterwards return to a state of rest.
If these agitations are repeated at the same point, there will result, as in air, a series of undulations analogous to those producing sound; and as in these there are observed successive and periodical alternations of condensation and rarefaction, corresponding to the alternations of direction which constitute the vibrations of a sonorous body, in like manner it will be easily conceived that the successive and periodical vibrations of luminous bodies might produce similar effects in luminous undulations: and again, as the succession of sonorous waves, when sufficiently rapid, produces on our ear the sensation of a continuous sound, the quality of which depends on the rapidity of the opposite vibrations, and on the laws of condensation and velocity that the nature of these vibrations excites in each sonorous wave, in like manner, under analogous conditions, the ethereal waves may produce sensations of light in our eyes, and different sensations in consequence of the variety of the conditions. Hence the differences of colours. In this system, the length of the luminous waves correspond to Newton's fits, and
