Features of Light and Darkness shows how the appearance of colored objects changes under varying conditions of lighting; that is, under different kinds and brightnesses of light. We see these effects every day, but not all are dramatic, and some are so familiar that we no longer notice them. Thus under a red light, pretty much everything looks red. You may also know that the colors of parked cars look odd at night under mercury-vapor or sodium-vapor street lighting. Or perhaps you have noticed that in dim light, red colors look much darker, compared to other colors, than in the full light of day. There are related phenomena that are less common, as well.

Features of Light and Darkness is designed to show such effects, but the app cannot reach out and set up colored spotlights in your room, or make the sun rise and set. So it simulates -- it calculates what your eyes would see if you were looking at a combination of lights and colors, and figures out how to present that result to you on the display of your portable device. Sometimes it cannot find a way to present the result, in which case you will see an error message announcing the problem, with boring technical details in the fine print.

The large rectangle on the main screen of Features of Light and Darkness presents a scene. Each scene displays a pattern that has been colored with two pigments, pigment 1 and pigment 2.

If you press the "Pattern" button, it will show you what pattern is in use, colored in black and white. In that display, black areas are the ones that will get pigment 1, and white areas will get pigment 2. You may also select a different pattern from among the ones displayed.

The upper part of the scene is illuminated with one light source -- the upper light -- and the bottom is illuminated with the lower light. You may press the buttons for the various light sources and pigments to see which ones are in use, or to select others.

Light sources and pigments are shown as spectra. Each light source is shown as a stylized representation of the spectrum -- the rainbow of colors -- that you would see if you passed a beam of that light through a prism. Each pigment is shown as the rainbow you would see if you reflected white light off a thick coating of that pigment and then through a prism.

The vertical sliders on either side of the scene move the boundary between the upper and lower light source. There is a narrow region in which the lights are mixed as they interchange. In the scene displayed when Features of Light and Darkness starts, the effect of moving the vertical sliders is particularly marked, and perhaps particularly mysterious, because the background stays the same color while the lettering changes from a slightly greenish yellow to black. We will resolve this mystery elsewhere herein.

The long horizontal sider just below the scene display adjusts the overall brightness, and simulates how human color vision changes as it gets dark: As the overall brightness declines, the relative brightness of colors changes. Speaking in general, reds, yellows and purples get darker, compared to blues and greens. The "Purkinje Effect" scene shows the relative darkening of red. These effects also mean that at low light levels, the human eye can distinguish certain combinations of colors that appear the same in full daylight: Try the scenes "Twilight Ink" and "In Darkness".

Incidentally, if you are having trouble seeing what is going on in a scene at low brightness, remember that you can simply open the "Settings" app on your device and increase the overall screen brightness. I did not want to have too high an actual brightness when the brightness slider was set to a low value, because these effects do happen only in truly dim light, but you are allowed to use "Settings" to cheat. I won't tell.

The short horizontal sliders adjust the relative brightness of light sources and pigments. There is one confusing point about how that works: The display of your device can only get so bright. So once Features of Light and Darkness has made the brightness of a particular area as bright as it can possibly get, the only thing it can do, if you want an even higher relative brightness, is to make everything else dim, and that is what it does.