The x-ray technique that Rae was using is capable of seeing through layers of paint to discover any secrets that lie beneath them. And in this case, Rae uncovered a sketch done in lead-white paint, a medium that the x-rays do not penetrate.
And the subject of the sketch, which had been painted over with the portrait of Maitland, was one of Scotland’s most iconic and tragic historic figures, Mary Stuart. She’s perhaps best known as Mary, Queen of Scots. Rae was able to confirm this by comparing the drawing to other contemporary images of Mary.