The scene named Features of Light and Darkness demonstrates two different kinds of light sources -- different spectra -- that appear identical to the naked eye, yet cause a single pigment to appear differently. In this scene, the upper light source is an orange light that is "pure", in the sense that if you sent it through a prism, the rainbow that it produced would have just one band, in the orange. The lower light source also is orange, but it is composed of a mixture of red and green light; that is, its rainbow would have a bright band in the red and one in the green. The pigment that forms the background for the lettering -- the "paper", so to speak -- is pure white, so when you look at it you see the light coming back at you unchanged. The pigment in the ink of the lettering is a very pure greenish yellow -- it reflects only a narrow band of light that is just a little toward the green from the pure yellow part of the rainbow. The upper light has plenty of that color in it, which reflects back from the ink pigment and shows it as yellow with a touch of green. The lower light has none of that color, so that when it shines on the ink nothing reflects back, hence that part of the writing looks black.

In Darkness depicts a creature whose coloring is indistinguishable from the background color in full daylight, but who can be seen when it is getting dark. Move the horizontal slider to the left until he appears, and if you are having trouble seeing him, go to the "Settings" application of your device and raise the brightness of the screen there. The upper light is the same as the lower light in this scene. What makes the invisibility possible is that the mechanism of human color vision is different in bright light and in dim light. The same colors are also used for Twilight Ink.

I think this scene would make great wallpaper or a great painting in a bedroom. It would be very scary when night was falling.

The Purkinje Effect is named for a scholar who liked to meditate in a flower garden. One evening he noticed that although the red petals of some of the flowers in the garden were much brighter than the green foliage in full daylight, they appeared darker than the same foliage in twilight. He concluded, correctly, that the human mechanism of color vision is different in dim light than in bright light. This scene illustrates that effect. Move the horizontal slider to see it. Try some other pigments for the flowers; oranges and yellows will also show the effect. The upper light source is the same as the lower one in this scene.

Twilight Ink conceals a secret message written with a particular color of green ink on a particular color of green paper. The two colors look alike in bright light, but they are distinguishable at low light levels. Move the horizontal slider to see it happen. The upper light source is the same as the lower one in this scene. What makes the invisibility possible is that the mechanism of human color vision is different in bright light and in dim light. The same colors are also used for In Darkness.

In Under a Red Light, in the upper part of the scene, almost everything looks red. The lower light source is white. Try some different pigments for the colors, and some different light sources for the upper one.