I love QI.
What I believe is the correct answer:
Leonardo used a technique common at that time, having the eyes angle a certain way (slightly down and to the right I /think/) so that it appears she's looking 'at' you from wherever you stand in the room.
What I believe is the correct answer:
Leonardo used a technique common at that time, having the eyes angle a certain way (slightly down and to the right I /think/) so that it appears she's looking 'at' you from wherever you stand in the room.