The controversy between the wave theory and the emission theory of light early in the nineteenth century has been a subject of numerous studies. Yet many is sues remain unclear, in particular, the reasons for rejecting Young's theory of light. It appears that further progress in the field requires a better grasp of the overall situation in optics and related subjects at the time and a more thorough study of every factor suggested to be of importance for the dispute. This book is intended to be a step in this direction. It examines the impact of the concept of interference of light on the development of the early nineteenth century optics in general, and the theory of light, in particular. This is not a his tory of the wave theory of light, nor is it a history of the debate on the nature of light in general: it covers only that part of the controversy which involved the concept of interference. Although the book deals with a number of scientists, scientific institutions, and journals, its main character is a scientific concept, the principle of interference. While discussing the reasons for accepting or rejecting this concept I have primarily focused on scientific factors, although in some cases the human factor is examined as well. The book is a revised Ph. D. dissertation (University of Minnesota, 1984) writ ten under Alan E. Shapiro.