But mechanics, albeit the oldest, is yet only one of the physical sciences. The great advance made in the mathematical formulation of optical and electromagnetic theory during the nineteenth century revived the hope of discovering absolute motion in space by means of the laws derived from this theory.]264 [Newton had supposed light to be a material emanation, and if it were so, its passage across "empty space" from sun and stars to the earth raised no problem. But against Newton's theory Huyghens, the Dutch astronomer, advanced the idea that light was a wave motion of some sort. During the Newtonian period and for many years after, the corpuscular theory prevailed; but eventually the tables were turned.]* [Men made rays of light interfere, producing darkness (see page 61). From this, and from other phenomena like polarization, they had deduced that light was a form of wave motion similar to water ripples; for these interfere, producing level surfaces, or reinforce each other, producing waves of abnormal height. But if light were to be regarded as a form of wave motion--and the phenomena could apparently be explained on no other basis--then there must be some medium capable of undergoing this form of motion.]135 [Transmission of waves across empty space without the aid of an intermediary material medium would be "action at a distance," an idea repugnant to us. Trammeled by our tactual, wire-pulling conceptions of a material universe, we could not accustom ourselves to the idea of something--even so immaterial a something as a wave--being transmitted by nothing. We needed a word--ether--to carry light if not to shed it; just as we need a word--inertia--to carry a projectile in its flight.]231 [It was necessary to invest this medium with properties to account for the observed facts. On the whole it was regarded as the perfect fluid.]235 [The ether was imagined as an all-pervading, imponderable substance filling the vast emptiness through which light reaches us, and as well the intermolecular spaces of all matter. Nothing more was known definitely, yet this much served as a good working hypothesis on the basis of which Maxwell was enabled to predict the possibility of radio communication. By its fruits the ether hypothesis justified itself; but does the ether exist?]231 THE ETHER AND ABSOLUTE MOTION