This is something that i noticed.i.e., it happen to me, when i went to my teacher to ask him why it happens he actually didn't believe me, i guess maybe because he was a physics teacher! Consider a room with\ all the lights switched off, let their be a far off light source which is very weak. So you can't actually see anything clearly but you can make out the boundaries of things around you, faintly. What i observed was that when there was a particular object in front of me, or even if the dim far off light source is in front of me and i tried to look straight at it i could not see it, but if i moved my line of sight away from the source, i could see it. Summary, could not see it when i look right at it but could see it in the peripheral part of my vision, t when i looked at something else. why does that happen? I thought about this for a few years actually because no one believed me! I hope you guys do! I actually came up with a lame excuse, it was that, maybe because when i look straight at a thing i'm sensing it mostly by my fovea which is loaded with cones, i just assumed that as there are so many cones there would be lesser rods, hence bringing down my sensitivity at low intensities of light! Is that right? Also try doing this and see if you observe something similar to what i tried to describe above!