What is a dichroic material?

 A dichroic material causes visible light to be split up into different wavelengths, that manifest as colours.

Bill Gilbert on Magnetism (De Magnete)

William Gilbert was an English physicist and physician (for a few years he was the physician of Elizabeth I) and also a natural philosopher.  In 1600 he published a book on magnets called De Magnete. He was born in the historic English market town of Colchester in Essex and studied medicine at St John's College, Cambridge.  His experiments concluded that the Earth is magnetic and this was the reason compasses point North and he further argued that iron was at the centre of the Earth. Francis Bacon, philosopher and former Lord Chancellor, attacked Gilbert's conclusions on magnets arguing he had "built a ship out of a shell".