Human Visual Purple and Absolute Threshold

Abstract

I HAVE just read the interesting communication by Drs. Crescitelli and Dartnall on human visual purple (rhodopsin)¹. The last paragraph of that article may give rise to the impression that new evidence has been brought forward, as a result of which the work of Hecht, Shlaer and Pirenne² on the minimum number of light quanta necessary for vision might have to be considerably revised. Such an impression would, I think, be mistaken. Crescitelli and Dartnall were able to repeat and confirm König's³ measurement, made in 1894, of the optical density of the rhodopsin extracted from a human retina. On such a basis König calculated the percentage p of the light incident on the human retina which should be absorbed by the rhodopsin contained in the living rods, and found p = 4 per cent for λ = 0.51 µ. Crescitelli and Dartnall now find p = 3.5 per cent. König's result was quoted and taken fully into account in the above paper by Hecht et al. Crescitelli and Dartnall's work, however valuable in other respects, affects but little the outcome of the discussion.