A Photochemical Reaction in the Human Retina

The way in which the disturbance of a colour match by adaptation is affected by the brightness of the adapting field is investigated. It is inferred from the results that the disturbance is due to a photochemical reaction in a pigment for which the apparent value of the product of quantum efficiency and molar decadic extinction coefficient at λ 0.574 μ is about 420 000. Reasons are given for believing that the true value is probably lower. Three hypotheses concerning the nature of the pigment are discussed, and it is concluded that the most likely of them is that it is the receptor pigment of the `red-receptive' mechanism of trichromatic theory. If so, this must have the very high optical density in the receptors of about 0.98.