Haidinger's brushes

You heard it here first. The human eye is capable of detecting whether linearly polarized light is polarized in the parallel or the perpendicular direction. Your brain simply erases the characteristic pattern that your eye produces when looking at such light. But you can train yourself to see the pattern:

The pattern is yellow, and bow-tie shaped. Some people report seeing a similarly shaped blue bow-tie at a right angle to the yellow. It is about an inch across. And appears at the center of your visual field.

If you have a pair of polarized sunglasses, you can teach yourself to see the brushes by staring at a linearly polarized source of light and rotating the glass at a rate of about 90 degrees/sec (to compensate for the rate at which your brain erases the pattern). Linearly polarized sources of light are a little hard to come by if you don't work in a laboratory, but a blue portion of sky at a right angle to the sunrise or sunset is linearly polarized and will allow this to work.

I can't find anything about Haidinger: discovered the phenomenon in either 1844 or 1846; might have been a Croat. And I don't know what sort of brushes they had back then wherever Haidinger lived, but I can't see it; the pictures don't even look a lot like foxes' brushes. A bow-tie or hourglass shape describes it better to us, a vertical yellow hourglass with possible bluish spots to left and right.

It is only certain human cells that can detect polarization (they are dichroic): possibly ones in the fovea, in the centre.

Full polarization vision is common in insects. Horizontal surfaces like leaves and sheets of water reflect one direction of polarization, and the sky transmits a perpendicular one. Bees would get a confusing shimmer, constantly changing, if they saw all polarized light, so it appears to be restricted to the part of the eye looking upward, so they can navigate by the sun (even when it's hidden), and to the ultraviolet, so it doesn't radically affect colours. (Bees' primary colours are blue, green, and ultraviolet.)

The light-sensitive molecule in eyes is rhodopsin. In our eyes it is randomly aligned, so does not react preferentially to any direction of light. But an insect's compound eye is arranged as a packing of tubes. Any one tube has a specific alignment. But in the bee, most of them are twisted anyway, so the alignment is diffused or re-randomized. Only in the upper (skyward) parts of the ommatidium do they have tubes of nine cells (microvilli) that preserve a preferential alignment: one of the three ultraviolet-sensitive cells is closer to the alignment of another, so there is a reinforced sensitivity for polarization in the ultraviolet colour.

Well if this isn't Node what you don't know, I don't know what is.