A bit off the refraction topic but Chuck O'Connell expressed interest in dials that use polarisation. There was an interesting article recently in one of the BSS Bulletins about using Sellotape as a polarising medium to make a dial that would indicate hours even when it's cloudy - it uses the fact that the background light from the sky is polarised.
I forget the issue but it can probably be found in the list in 'Sundials on the Internet'. Patrick
