"traced their outlines upon my mind"

This passage reveals the link Proust wants to establish in the reader's mind between the memory and art. The narrator's memories of Combray, traced on his mind, are compared to the reproductions of art that his grandmother used to show him. Proust believed that the past could only be regained through art, just as an engraving can capture the essence of a work of art which has now faded and decayed.