The acuity of the viewing eye decreases in intensity from fovea to periphery under daylight conditions but not when the light is dim. This is because, among the photoreceptors at the back of the retina, the closely-packed cones are sensitive both to hue and to fine detail under conditions of high illumination (thus providing daylight, photopic vision), whereas the ever more sparsely-distributed rods, placed peripherally in relation to the cones, are less sensitive to detail and blind to hue yet provide monochrome (or scotopic) vision in dim light. This is why, under conditions of low illumination, we see things more distinctly if we don't look directly at them (the so-called Arago phenomenon), which tells us that, if we fixate near the center of any painting, peripherally-placed elements will come into attention as the light fades. Bonnard helps along this phenomenon by painting figures and objects in the margins of his paintings a similar color to the ground, thus making them difficult to see under normal conditions. And he often makes use of the varying appearance, under varying light levels, of long- and short-wavelength spectral colors.
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