The discovery of vitamin A’s visual function presents a good example in the history of science of the step-wise nature of discovery, according to George Wolf. “The story started with the crucial observation that light bleaches the color of the retina. The properties of this process in relation to vision were then exhaustively explored. The connection was made to dietary vitamin A deficiency and night blindness; vitamin A was found to be present in the retina. The final step was the discovery of the involvement of vitamin A in a cyclic visual process and its explication at the biochemical and finally the molecular level.”

George Wolf: The discovery of the visual function of vitamin A.  J Nutr. 2001 Jun;131(6):1647-50. (full-text)