Historian Dr Janina Ramirez unlocks the secrets of a centuries-old masterpiece in glass. At 78 feet in height, the famous East Window at York Minster is the largest medieval stained-glass window in the country and it was the creative vision of a single artist - a mysterious master craftsman called John Thornton, one of the earliest named English artists.  The East Window of York Minster is far more than a work of artistic genius, it is a window onto the medieval world and the medieval mind - telling us who were once were and who we still are, all preserved in the most fragile medium of all.