The waves of light are not anything mechanical or material, but are something electrical and magnetic—they are, in fact, electrical disturbances periodic in space and time, and travelling with a known and tremendous speed through the ether of space. Their very existence depends upon the ether, and their speed of propagation is its best known and most certain quantitative property.

The Ether of Space, p. 13 - The Ether of Space (1909)