Inspired by Tim Hamilton's facebook post I played a bit with my old bit of silfurberg this weekend
In particular, I was curious to test whether there was some plausibility to birefringent calcite being the "sunstone" of the Saga's.
This evening, about an hour before sunset, we had nice thick rain clouds moving from Ohio, so, as I had promised the Astronomers fb group, I popped out to check what I could see.
After a short time playing with rotation, looking with peripheral vision and doing some slow scans, I gave the biggest piece to my daughter (who I am reasonably sure has not read this stuff, but did hear us talk about it at dinner sunday) and asked her what she saw.
After playing with orientation etc, much as I had, she told me she thought she saw a coloured spot between the two trees.
I asked her what colour, and she said yellow, with blue fringes.
This was consistent with the description of Haidinger's Brush, consistent with what I saw, and where she pointed was consistent with where I knew the Sun actually was, about 15 degrees above the horizon in the west.
ED: per comments, though it cannot have been Haidinger's Brush
The colouration was surprisingly clear and well localized.
It even came through on the iPhone picture.
Not a quantitative analysis, but turning the crystal appeared to give a different axis of elongation to the yellow smudge with blue fringe (blue response of phone is poor, don't see it in picture)
I tried a few simple experiments:
scanning the sky, I thought I saw some of this yellowish smudge across a fairly broad arc;
looking at a lightly coloured grey sidewalk in same ambient light, I saw no yellow colouration;
looking at a matte bright white surface (bed cover) in bright artificial light, I saw no yellow colouration.
Naively, #CaveatTheorist, this seems consistent with the tales: the sunstone permits the naked eye to see the polarization pattern of the cloudy sky (op cit) and peaking up on the response give you a good localization of the Sun. Not a real detection per the sagas.
Further, the "glint" I saw yesterday was almost certainly an internal reflection of stray direct sunlight getting through the lighter cloud cover.
But, looking closely, what I did see is that to get a good colouration the rotation of the crystal was aligned with an internal fault plane, which would align on the principal axis of the crystal.
That is interesting and potentially confirms that this really is a polarization effect, that it is real and useful.
Still not sure I'd stake my life on it in a small wooden boat in the North Atlantic, but it might work reasonably with a bit of practise.
ED: I remain intrigued that there is an effect, and in my copious spare time want to look more closely at the measured sky polarization in cloudy conditions.
Were I to speculate, as a theorist, without actually doing any quantitative analysis, as theorists sometimes do, I'd say that the presence of a clean fault plane in the crystal seems to be driving whatever effect I see - the colourations are clearly associated with a preferred orientation of that plane - whether that is just marking the principal axis of the crystal, or whether actual optical properties of the internal fault are relevant is not clear.
Also, since I am a couple of hundred miles from the sea I always know the cardinal directions when doing this, so there is huge potential subliminal bias.
Good fun though.
You shouldn't be able to photograph Haidinger's Brush because it is a perceptual phenomenon of the retina. If you can photograph it, then it' might be residual coloration in the crystal, or possibly some wavelength dependence of the polarized light (yellow cast?) Wavelength dependence seems a little unlikely.
Polarization of scattered light from the sky ought to be strongest at 90 degrees from the sun, not in the direction of the sun. I am a little skeptical that polarization survives multiple scatterings in a thick cloud, but have not gone out and looked at clouds through a polarizer to test this (this would be a good check on any sunstone/calcite phenomena). Maybe there is still a statistical excess.
There is evidence that some migratory animals can sense the polarization of skylight and use it as a cue, although googling (try "polarized light birds", or pigeons) suggests that some of the results are disputed.
I found that I can see Haidinger's brush pretty well on a white laptop LCD screen; have not tried it in the sky yet. I'm not sure that the use of the sunstone would rely on Haidinger's brush, since the brush is a directly visible effect of polarized light rather than requiring the sunstone to detect polarization.
Further thought: I'm skeptical that the sunstone (or even a polarizer) allows one to sense the direction of the sun through thick cloud cover. If Iceland spar was really used for direction finding, I think it's possible that it was used to sense polarization when the sun was low on the horizon and obscured by haze or low cloud, which would be pretty common in high latitudes. Or possibly that it was used in conditions of flat light or sector whiteout, where even a thin cloud cover could combine with the featureless sea to cause disorientation. For more informed speculation you'd want to consult an actual sailor, which I am not.