Treatise on Light

OF LIGHT. CHAP. I.

...particles of the ether to be of a material so nearly approaching perfect hardness and of an elasticityHuygens uses the word ressort (spring). In 17th-century physics, this refers to the ability of particles to instantly snap back to their original shape after being compressed, a property we now call elasticity. so prompt as we desire. It is not necessary here to examine the cause of this hardness, nor that of the elasticity, as such a consideration would lead us too far from our subject. I will say, however, in passing, that one can conceive that these particles of the ether, notwithstanding their smallness, are still composed of other parts, and that their elasticity consists in the very rapid movement of a subtle matterA hypothetical substance composed of extremely fine particles that Cartesian scientists believed filled all voids and transmitted forces through the universe. which traverses them from all sides and constrains their texture to arrange itself in such a way that it gives the most open and easy passage possible to this fluid matter. This agrees with the reasoning that René Descartes (1596–1650), the influential French philosopher and mathematician who proposed that light was a pressure transmitted through a medium.Mr. Descartes gives for elasticity, except that I do not suppose pores in the form of round, hollow canals as he does. And one must not imagine that there is anything absurd or impossible in this; it is, on the contrary, quite credible that it is this infinite progression of different sizes of corpusclesSmall particles or physical atoms. and the various degrees of their speed which Nature uses to produce so many marvelous effects.

But even if we were ignorant of the true cause of elasticity, we see constantly that there are many bodies which possess this property; and thus there is nothing strange in also supposing it to exist in small invisible bodies like those of the Ether. If one wishes to seek any other way in which the movement of light is communicated successively, one will find none that suits the uniform progression—which seems to be necessary—better than elasticity. This is because if this movement slowed down as it is shared among more matter while moving away from the source of light, it could not preserve such great speed over vast distances. But by supposing elasticity in the ethereal matter, its particles will have the property of restoring themselves equally fast, whether they are strongly or weakly pushed; and thus the progress of the li—