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, shoemaker's 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 in 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
- ↑ The 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.
