I've always wanted to build a birefringent sun dial, "I only count the cloudy hours": https://en.wikipedia.org/wiki/Iceland_spar#Uses https://en.wikipedia.org/w

I've always wanted to build a birefringent sun dial, "I only count the cloudy hours": https://en.wikipedia.org/wiki/Iceland_spar#Uses https://en.wikipedia.org/wiki/Iceland_spar#Uses