It can be hard to make out the dog in the lower right-hand corner of Rembrandt’s monumental group portrait, “The Night Watch.” The paint has faded over time, and the dog falls into shadow, so its outline appears like just a blotch on the canvas. Yet for those in the know, the dog is a beloved feature that parents delight in pointing out to their children at the Rijksmuseum in Amsterdam. On Tuesday, researchers at the museum announced that they had discovered Rembrandt’s inspiration for the dog: a drawing by an artist named Adriaen van de Venne that was copied by another artist, François Schillemans, as an engraving for the title page of a book. “We don’t know if Rembrandt used the drawing or the engraving,” said Anne Lenders, a curator of 17th-century painting at the museum. “But we know that he must have been familiar with one of the two.”