How was this painted 370 years ago?

Diego Velázquez painted Las Meninas in 1656, near the end of his three decades as court painter to King Philip IV of Spain. But the painting has never fully given up its secrets. For over 370 years, historians have argued about the mirror on the back wall, the identity of the canvas Velázquez is painting, and whether the king and queen are standing in the room or only in reflection. Using X-ray analysis, conservators at the Prado found something no one could see on the surface. Across the entire 11 foot canvas, Velázquez barely changed a single brushstroke, except in one place. This is the story of what science found hiding inside one of the most argued-over paintings in history. #velazquez