Abstract

During the first four decades of the nineteenth century a debate raged over the nature of light. Following proposals of Isaac Newton made early in the eighteenth century, many physicists accepted the theory that light is composed of tiny particles subject to mechanical forces (see newton). At the beginning of the nineteenth century Thomas Young and Augustin Fresnel revived a competing theory originally suggested by Christiaan Huygens in the seventeenth century, according to which light consists not of particles, but of waves in a medium called the ether. In both cases theorists postulated entities ‐ particles and waves in an ether ‐ that could not be observed. Yet theorists on both sides gave arguments for the existence of these entities and the properties they ascribed to them. They did so on the basis of what they could observe. How was this possible?