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Abstract
THE Purkinje shift is the displacement of the maximum sensitivity of the eye towards the blue end of the spectrum at low levels of ambient illumination. It occurs in a wide variety of vertebrates, including some which possess the porphyropsin system of freshwater fishes in place of the rhodopsin system of sea-water fishes and land vertebrates; but in spite of this widespread occurrence, there is no satisfactory explanation for the advantage derived from evolving different photosensitive systems with different spectral sensitivity curves for use at high and low levels of illumination. An explanation is not to be found in the different distribution of energy in the incident light in day-time and at night, for the maximum energy of light from the Moon is slightly to the red side of the maximum for light from the Sun. Le Grand1 suggests that rhodopsin was evolved by deep-sea fishes, for the light which penetrates deep into the sea has a maximum energy close to the peak of the absorption of rhodopsin, but a shift of the spectral sensitivity curve to the blue may enable the eye to achieve greater sensitivity for a quite different reason.
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