From Physorg.com:
“If you were to look at these paintings, you would assume that Venus is admiring her own face, because you see her face in the mirror. Your viewpoint, however, is rather different from hers; if you can see her in the mirror then she would see you in the mirror.”
Participants were also asked to estimate the image size of their head as it appears on the surface of the mirror. They estimated that it would be a similar size to their physical head. However, participants based their answer on the image they saw inside the mirror rather than on the image on the surface of it. They failed to recognise that the image on the surface of the mirror is half the size of the observer because a mirror is always halfway between the observer and the image that appears inside the mirror.
More here.