We see the world with two kinds of vision.

By day, vision is “photopic.”  We see in color, using a trio of cone cells sensitive to blue, green, and yellow light.   At night vision is “scotopic.”  We see with a single cell type— called rods— in shades of gray.

Between these two is the “mesopic” range.  Dusk and dawn, rain and fog.  Vision is a mix of rods and cones, and they interact in all sorts of weird and nonlinear ways.  Colors are duller but can seem more vivid.  This is the light I like best as a painter.

One thing that happens as the light gets dimmer is that we become more sensitive to blue versus yellow light.  This is called the “Purkinje shift” and was described in 1825.  There is a beautiful blue-gray color that appears in the distant trees around sunrise that I always try to capture on my camera.  And then I am disappointed when it shows up as an uninspiring dull green.  One foggy morning I tried to catch it with paint, and I think I was a little more successful (although the painting is still not as luminous as the actual trees).  When I did the painting I had no idea why the color was there.