Colors form a spectrum with red at the long wavelength end and blue and mauve at the short wavelength end. Among the photoreceptors, only a tiny percentage of the foveal cones are responsive to short-wave colors like mauve and blue and are overwhelmingly responsive to the long-wave yellows and reds which compose daylight. Thus, as the light fades, and vision shifts from the foveal cones to the peripheral rods, short-wave colors increase in brightness and, therefore, increase visibility relative to long-wave colors.
This is known as the Purkinje shift, named for the physiologist Jan Evangelista Purkinje (1787-1869), who first reported the phenomenon. Because Bonnard used almost the full spectral color range in his paintings, the phenomenon is very noticeable in his work. Additionally: the two ends of the spectrum cannot be concurrently focused by the eye. The eye is normally focused for the yellow light so abundant in the world, an effect Bonnard helped along by loading his pictures with yellow. ("One can't have too much," he would say.) Thus, short-wave light is permanently out of focus under daylight conditions; therefore, substances indicated in mauves and blues beside or within areas of long-wave colors will seem distanced and remote in daylight vision, as frail Marthe often does in Bonnard's paintings, most notably those that show her water-glazed body.
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