Bill Arnold and calorimetric measurements of the quantum requirement of photosynthesis-once again ahead of his time.

The approach of photocalorimetry to decide on the true quantum requirement of photosynthesis - one of the main issues of the research in the first half of the century and a source of a bitter debate - is described. Bill Arnold's original approach to get into the true answer is reflected from the point of view of present day calorimetric techniques.