Steve Mould's nerdy blog

I discovered an illusion when I opened my eyes this morning. I happened to be looking in the direction of 2 stacked pillows and the dark region where the two pillows met appeared to expand for about a second. I noticed it worked better when I was looking at a bright part of one of the pillows instead of the dark region itself.

I've recreated the effect below. To make it work, close your eyes for 10 seconds. When you open them, immediately look at the grey dot. The dark region in the middle should appear to expand for about a second:

I'm not sure if this has been done before or even if it works for anyone but me! Let me know if it works for you.

My theory is that when you first open your eyes, your pupils are dilated, so a lot of light is coming in. As a result, the dark patch looks less dark. In the fractions of a second after you open your eyes, your pupils contract. Less light gets in, so the dark patch gets darker. Because there is a gradient from the middle outwards, that darkening could also be interpreted by your brain as the dark patch expanding.

It might be that other things are happening besides your pupils contracting. Your brain does all sorts of adjustments to the information coming from your eyes.

Film camera operators talk about white balancing before taking a shot. It's where they make adjustments to their camera so that things that are supposed to look white actually do end up looking white. Your brain does some pretty intense white balancing too and it leads to a phenomena known as colour constancy. There's also brightness constancy going on in your brain. And if this brightness constancy takes any sort of time to kick in, that could also be an explanation for the effect in this illusion.

Here's a quick video I made of my self siphoning beads:

The chain is the stuff from vertical blinds. The thing you pull on to open and close them. Searching for metal 4.5mm bead chain on ebay gets you the right sort of thing. That also bring up the linkages you'll need to buy to make the chain really long.

The reason it self siphons is that the chain pulling down outside the beaker is longer (and heavier) than the chain pulling down on the inside of the beaker.

The reason the chain loops up higher than the rim of the beaker is a bit of a mystery. I think it's to do with the fact that the beads are changing direction and it can't do that instantaneously as that would represent infinite acceleration. So a big loop is formed. I don't know, something like that.

I made a video to show you the cool way a rod of aluminium vibrates when you hit it:

I was giving my Illusions talk at the Royal Institution and had a few minutes at the end to film this video about purple.