Mere Christianity Chapter 7 ("Let's Pretend") Book IV: Reference to Story of Mask

In Mere Christianity Chapter 7 ("Let's Pretend") Book IV, Lewis writes that there is a story > about someone who had to wear a mask; a mask which made him look much > nicer than he really was. He had to wear it for year. and when he took > it off he found his own face had grown to fit it. He was now really > beautiful. Does anyone know what this story is? Thanks