The Undisturbed Propagation of Waves in the Ӕther
Confirmations of the hypothesis are both numerous and exact. It is indeed a different matter if one could wish to understand this rectilinear propagation. What is it, then, which is finally propagated in straight lines? In order to answer this one should have to obtain an insight into the mechanism of the propagation of light. For many centuries it was not decided whether light required time in order for it to be propagated, or whether it moved instantaneously through space. It would appear that in ancient times the general assumption was that it travelled instantaneously. The experiences of daily life, and the experiments which were then possible, justified this view. The question concerning the manner in which light is propagated is very intimately connected with the problem of the nature of light itself. For the transmission will be essentially different according as to whether light consists of particles projected at a high speed, or of a progressive wave motion.