This is a detail from a 16th C copy of Rogier van der Weyden's Saint Luke drawing the Madonna. For the religiously literal-minded today, it must seem silly to imagine that one of Jesus's disciples was around as an adult to sketch His mother when He was a baby; but to the more mythically minded Middle Ages and Renaissance, what mattered was not chronology, but devotion. Luke the Evangelist and presumed author of the third Gospel was also believed to be an artist, ergo … a lot of symbolism packed into one painting. Well, leaving aside all the whole, huge topic of belief, resonance, and levels of interpretation, I come down to loving this detail for its exactitude in portraying an artist at work. Even better for the historical novelist is a splendid website post, History and Usage of Metalpoint Drawing.