Bucket science

’Look at the sky in a direction of ninety degrees from the sun, and you will see a yellow and blue cross, with the yellow toward the sun, and from the sun, spreading out like two foxes’ tails with blue between...if you do not see it.try another method. Look into a pail of water with a black bottom. and look down at the surface of the water on a day with a white cloudy sky.look at it with the head tipped on one side and then again with the head tipped to the other side, keeping your eyes on the water.do not do it fast or you will make yourself giddy.’
Lord Kelvin, addressing The Academy of Music, Philadelphia, US, 29/9/1884.

Right, forget the pail of water with the black bottom, this is what you do. First, get a flat screen monitor (rather than an older cathode ray tube display) and find a way to generate yourself a blank white screen. This can be a page in a word processing or painting programme - the key thing is that it must be white and the white area must be reasonably large and uninterrupted.

Done that? Right, now position yourself a comfortable distance from the screen, if anything maybe slightly further away than you normally are for general use. Close your eyes for a good 20 seconds and relax them so they are completely free from strain; then open them and stare directly at the screen.

Try to defocus somewhat, as if looking beyond the screen, and then tilt your head as far as you can to the left while maintaining concentration on one spot on the screen. Then tilt your head as far as you can the other way, quite rapidly (over the course of maybe a second or two) while maybe allowing the centre of your gaze to move slightly in the direction you’re tilting towards, all the time keeping focus behind the plane of the screen as much as possible.

If you persist with this unorthodox routine you should start noticing an interesting effect. Actually, if you’re doing this in a shared office the most pronounced effect will be mutterings from colleagues rapidly followed by a visit from the occupational health people.

That aside, what you’re looking for are splodges of yellow at the point of focus, indistinct at first but becoming more pronounced and structured with practise. Little golden hourglasses with, if you’re lucky and/or persistent, purple coloured wings at the waist.

See them? If not, practise will make perfect; if you can see them then congratulations, you have proved you can detect polarised light with the naked eye. For the purple and gold hourglass patterns are non other than Haidinger’s Brush, a visual effect first documented by Wilhelm von Haidinger in 1846.

It seems the key to it stems from the fact that polarised light (and this is where the flat screen as source rather than cathode ray tube becomes important) will interact with the dichroic pigment lutein, which is present in the retina. Aspects of this explanation are included in an excellent website devoted to the effect (see below).

I have to admit that attempts to graduate from computer screens to the open sky have proved fruitless for me. Lord Kelvin’s bucket technique was easily arranged - cloudy skies are the East Midland’s forte after all, and I had a bin liner handy - but to no avail. Direct observation of the blue sky near dusk - supposedly the most favourable scenario - proved equally unrewarding. Half an hour of persistent effort, eyes fixed on the sky and head rotating slowly, resulted in nothing more than a cricked neck and the neighbours calling the police.

Still, I’m sure there are readers who can tell me where I’m going wrong.

Paul Kelly