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details, a crowd of analogous phenomena observed to attend reflexion in thick plates, which, in fact, in his experiments, exceeded by as much as twenty or thirty thousand times those on which the calculations had been founded; moreover, applying the same reasoning to the integrant particles of material substances, which all chemical and physical phenomena show to be very minute, and to be separated, even in the most solid bodies, by spaces immense, in comparison of their absolute dimensions, he was able to deduce naturally from the same principles, the theory of the different colours they present to us, a theory which adapts itself with a surprising facility to all the observations to which those colours can be submitted. The number and importance of those applications account sufficiently for the case which Newton took with his experiments on the rings; I am sorry to be obliged to confine myself here to the bare indication of those fine discoveries.
Another explanation of the coloured rings on the hypothesis of undulations. Dr. Young's principle of Interferences.
If light be really a material substance, Newton's fits are a necessary property, because they are only a literal enunciation of the alternations of reflexion and transmission which coloured rings present; but if light be otherwise constituted, these alternations may be accounted for differenly.
Descartes, and after him Huyghens, and a great number of natural philosophers, have supposed that the sensation of light was produced in us by undulations excited in a very elastic medium, and propagated to our eye, which they affect in the same manner as undulations excited in the proper medium of the air, and propagated to the ear, produce in it the sensation of sound. This medium, if it does exist, must fill all the expanse of the heavens, since it is through this expanse that the light of the stars comes to our eyes: it must also be extremely elastic, since the transmission of light takes place with such extraordinary velocity; and at the same time its density must be almost infinitely small, since the most exact discussion of ancient and modern astronomical observations, does not indicate the least trace of resistance in the planetary motions. As to the relations of this medium with earthly bodies, it is plain that it must pervade them all, for all transmit light when
