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When I'm looking at an old CRT monitor or a worn fluorescent lamp, but not directly in the center of my gaze, the light from it seems to flicker. When I focus my gaze onto the monitor or lamp, the flickering stops.
Why can I see a light flicker only when it's in my peripheral vision?
Your retina contains both rods and cones. Cones are color sensitive, slow, and concentrated near the center of your field of vision. Rods are "light" sensitive, fast, and concentrated near the periphery. You want to be able to respond quickly to a threat "in the corner of your eye" without needing to see the color of the threat.
This is nicely explained by this diagram (source: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/imgvis/retina.gif)
So when you see a CRT in the corner of your eye, your rods respond quickly enough that you can tell the light of the CRT is flickering. As you turn your gaze towards it, you get the higher resolution, color rendition of the cones - and lose the temporal resolution.
