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 Royal 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 Hdinburgh 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 rigidity, 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 
