From Neil Downie
I found Philip Ball’s account of the search for Viking “sunstones” (21 March, p 40) fascinating. You can easily try out something similar yourself. First find some polarised glasses, such as those 3D specs from the cinema. Now make yourself a small square with one to six layers of Sellotape randomly stuck together.
If you look through the glasses at the square, backlit by a computer screen set to display a uniform light blue or white, you will be surprised to find that the square looks like a prettily coloured Mondrian painting, with different colours depending upon the number of overlapping layers. Now go outside and look at the square against different parts of a cloudy sky. You will find that the Mondrian is lifeless near the sun or opposite, but springs into nice colours at right angles.
The effect works nearly as well with a piece of calcite. Maybe Viking sailors made their own Mondrians out of thin stuff like sheep gut or mica?
Basingstoke, Hampshire, UK
