THE STATUS OF THE ETHER 107
ferent from those that have now been held for a century. In order to cover all the different notions that have been held, without being so definite in making the ether a substance as was Maxwell, we need only ask the question, Since we know that light travels with a speed of about three hundred thousand kilometers per second, and takes about eight minutes to come from the sun, what is the state of the light after it has left the sun and before it has reached the earth? We reply, it is traveling through the ether. A similar definition was given by the late Lord Salisbury who said that the noun ether was the subject of the verb to undulate. But why undulations? The undulatory theory, as a successful explanation of optical phenomena, is just about a century old, and was propounded by Dr. Thomas Young, in two Bakerian lec- tures before the Eoyal Society in 1801 and 1803. The reason that con- vinced Young, and later the scientific world, of the undulatory nature of light, was the fact of interference, or the production of darkness by the simultaneous action of two beams of light, carefully investigated by Young. These views were savagely assailed by Lord Brougham, in a scurrilous article in the Edinburgh Review, in which he says that "*it is a metaphysical absurdity, to assert that qualities can move in concentric surfaces." The violence of the attack may be seen from the quotation :
The long silence which he (Young) has since preserved on philosophical matters, led us to flatter ourselves, either that he had discontinued his fruitless chase after hypotheses, or that the Society had remitted his effusions to the more appropriate audience of both sexes which throngs around the chairs of the Royal Institution.
It is evident that Young had an excellent understanding of the analogy between sound and light waves, but he did not follow out the theory with the mathematical exactness bestowed upon it by Augustin Fresnel, whose superb researches, beginning in 1815, have made his name a classic of optical investigation. Both Young and Fresnel rec- ognized, as Huygens had not, the fundamental difference in the nature of waves of light and sound, namely, that since by turning the proper apparatus traversed by light about the direction of the beam as an axis, the light is capable of alternate extinction and transmission, the undulations must be transverse to the direction of propagation. Fres- nel introduced into his mathematical treatment certain mechanical principles, notably that one which we now call the conservation of energy, but he did not attempt to find a mechanical structure, in terms of properties of ordinary matter inertia and rigidit}', which would ex- plain the nature of the ether. This was done by George Green, who as- similated the ether to an elastic solid, which is capable of transmitting transverse waves in all directions with the same velocity. Unfortu- nately, such a solid transmits equally well longitudinal waves, like those