Escher also used a mirror to introduce a misleading element in the lithograph Still Life with Mirror.... this is a classic work for here we see Escher incorporates a clever optical illusion for the first time. The image reflected in the mirror is actually physically impossible. The mirror, which is tilted at a slight angle, is clearly in a room. We can tell this from the everyday objects placed around.. http://www.escherinhetpaleis.nl/escher-of-the-month/escher-of-the-month-current/?lang=en