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1. The visual pigment in a 5° circular patch of the living human retina 18° temporal from the fovea was studied with the Rushton retinal densitometer. The measuring light (570 nm) was selected to obviate artifacts from colour photoproducts.

2. The action spectrum of a 10% bleach agrees well with the action spectrum at absolute threshold for the same patch of retina. The quantized C.I.E. scotopic spectral sensitivity curve is a good description of both spectra. Therefore, the visual pigment studied must be human rhodopsin.

3. Its density has been estimated in five different ways. The results are in reasonable agreement. The optical density of human rhodopsin in vivo is about 0·35 (common logarithmic units) at its _max.

4. The photosensitivity of human rhodopsin in vivo was determined by studying its rate of bleaching in response to steps of monochromatic light exposed to the dark adapted eye, by measuring the amount bleached in the steady state by monochromatic lights as well as the amount bleached by 10 sec flashes of white light.

5. The results obtained by the different methods are in good agreement with each other and with previous estimates made by others using white light.

6. The photosensitivity of human rhodopsin in vivo [_max = 62,000 to 120,000 l./cm mole] is much higher than expected from in vitro measurements.

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