Application of Interference Methods 45
tary hypothesis to strengthen the theory; whereas the undulatory theory was competent to explain everything without the addition of extra hypotheses. Nevertheless, Newton objected to the undulatory theory on the ground that it was difficult to conceive that a medium which offers no resistance to the motion of the planets could propagate vibrations which are transverse (and we know that the light vibrations are transverse because of the phenomena of polarization), for such vibrations can be propagated only in a medium which has the properties of a solid. Thus, if the end of a metal rod be twisted, the twist travels along from one end to the other with considerable velocity. If the rod were made of sealing wax, the twist would rapidly subside. If such a rod could be made of liquid, it would offer virtually no elastic resistance to such a twist.
Notwithstanding this, the medium which propagates light waves, and which was supposed to resist after the fashion of an elastic solid, must offer no appreciable resistance to such enormous velocities as those of the planets revolving in their orbits around the sun. The earth, for example, moves with a velocity of something like twenty miles in a second, has been moving at that rate for millions of years, and yet, as far as we know, there is no considerable increase in the length of the year, such as would result if it moved in a resisting medium. There are other heavenly bodies far less dense than the earth, c. g., the comets, and it seems almost incredible that such enormously extended bodies with such an exceedingly small mass should not meet with some resistance in passing through their enormous orbits. The result of such resistance would be an increase in the period of revolution of the comets, and no such increase has been detected. We are thus required to postulate a medium far more solid than steel and far less viscous than the lightest known gas.
Light Waves and Their Uses
These two suppositions are possibly not as inconsistent as they may at first seem to be, for we have a very important analogy to guide us. Consider,, for example, shoemakers wax, or pitch, or asphaltum. These substances at ordinary temperatures are hard, brittle solids. If you drop them, they break into a thousand pieces; if you strike them (so lightly that they do not break), they emit a sound which corresponds to the transverse vibrations of a solid. If, however, we place one of these substances on an inclined surface, it will gradually flow down the incline like a liquid. Or if we support a cake of shoemaker's wax on corks and place bullets on its upper surface, after a time the bullets will have sunk to the bottom, and the corks will be found floating on top. So in these cases we have a gross and imperfect illustration of the coexistence of apparently inconsistent properties such as are required in our hypothetical medium.1 Nevertheless, it seemed impossible to Newton to conceive a medium with such incompatible properties, and this was, as stated above, a serious obstacle in the way of his accepting the undulatory theory. There were others, which need not now be mentioned.
For a long time after the various modifications that the corpuscular theory had to receive had been made, both theories were actually capable of explaining all the phenomena then known, and it seemed impossible to decide between them until it was pointed out that the corpuscular theory made it necessary to suppose that light traveled faster in a denser medium, such as water or glass, than it does 111 a rarer medium, such as air; while according to the undulatory theory the case is reversed. We may illustrate briefly the two cases: No matter what theory we accept, it is an observed fact that refraction takes place when light passes
iThe specialization of the undulatory theory known as the electro-magnetic theory does not remove this difficulty; for it is even more difficult to account for the properties of a medium which is the seat of electric and magnetic forces.