The Gradual Formation of the Concept of ‘Light Quanta’. The complex concept of ‘light quanta’ which made its first appearance in Albert Einstein’s 1905 paper on a “heuristic point of view” to cope with the photoelectric effect and other forms of interaction of light and matter, has a rich history both before and after 1905. Some of its semantic layers lead as far back as Newton and Kepler, others are only fully espoused several decades later, yet others initially increased, then (...) diminished in importance and finally vanished. Two historiographic approaches are discussed and exemplified: a) my own model of conceptual development as a series of semantic accretions, and b) Mark Turner’s model of ‘conceptual blending’. Both of these models are shown to be useful and will be further explored in my own efforts to come to grips with the complex process of concept formation. (shrink)