Morning illusion

Idling away the time while I waited for my tea to arrive this morning, I stumbled across an interesting illusion.

An illusion which seems to work best using one eye at a time, so not some artefact of binocular vision, in the way that being able to see through a finger held close to the nose is.

So, one holds one's two index fingers up together, nearly but not quite touching, a couple of inches in front of one eye or the other, against a bright, pale ground. Perhaps the sky, a pale coloured wall or a pale sheet of paper. With the result that the fingers look black against white.

As one brings the two fingers closer together, the sharp image of the inner surfaces of the fingers starts to break down. The inner surfaces of both fingers acquire a faint halo and various artefacts appear between them, in the grey area in the snap, perhaps a quarter of the top joint of one of the fingers in height. Perhaps a sliver of black, perhaps several slivers of black of graduated size, along the lines illustrated right. Sometimes blisters formed on one finger or the other, again along the lines of the three such illustrated right. Blisters which grew as the fingers moved closer together, eventually merging into the other finger, well before the fingers actually touched. At one point I thought I detected a frequency modulated sine wave spreading across the gap. All in all, quite complicated.

At which point I remembered about diffraction, once introduced at school and now introduced at reference 1. Perhaps this kicks in as the fingers get close enough together and the brain processing module which keeps the subjective images of things like fingers nice and tidy, the messy signal being turned in by the retinas notwithstanding, starts to break down. While a perfect brain, the sort of brain a robot of the future might have, would know how to make allowances.

Which diffraction might explain why I can't reproduce the effect on a Powerpoint slide, as Powerpoint does not do slits. A simpler explanation might be that one does not have enough control over the size of the gap.

PS: at one point I thought that the illusion might have been something to do with the fact that I was looking at the sky through a net curtain, with it being well known that net curtains generate all kinds of odd optical effects. But I have now discarded that theory.

Reference 1: https://en.wikipedia.org/wiki/Diffraction.