Researchers found that by expertly blending colours to exploit our peripheral vision, the shape of the subject's mouth appears to change according to the angle it is viewed from.

When viewed directly, the slant of the mouth is distinctly downwards, according to the research by scientists at Sheffield Hallam University and Sunderland University.

As the viewer's eye wanders elsewhere to examine other features, however, the mouth appears to take an upward turn, creating a smile that can only be seen indirectly, much like the Mona Lisa's.

The technique is called sfumato, and can be seen in both the Mona Lisa and La Bella Principessa.*

And while other artist's have attempted to use the same technique,  none have done so as expertly as da Vinci, the researchers claim.

'As the smile disappears as soon as the viewer tries to 'catch it', we have named this visual illusion the 'uncatchable smile,' researchers Alessandro Soranzo and Michelle Newberry of Sheffield Hallam University wrote in a paper published in the journal Vision Research.