The Mona Lisa's mysterious expression may have captivated the world, but hers isn't the only enigmatic smile Leonardo da Vinci created.

Researchers examining an earlier painting by the Renaissance master claim to have unravelled the painter's secret to creating an 'uncatchable smile'.

The study reveals how La Bella Principessa, painted by da Vinci before he completed the Mona Lisa in the late 15th Century, uses a clever trick to lure in the viewer.

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The study reveals how La Bella Principessa (right), painted by da Vinci before the Mona Lisa (left) in the late 15th Century, uses a clever trick to lure in the viewer. By expertly blending colours to exploit our peripheral vision, the shape of the subject's mouth appears to change according to view point, researchers found

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.

WHO WAS THE MONA LISA? Florentine noblewoman, Lisa Gherardini, is widely believed to be the model for Leonardo da Vinci's painting. Lisa Gherardini is thought to have posed for the painting between 1503 and 1506. Not much is knownown about Gherardini's life. Born in Florence and married in her teens to a cloth and silk merchant who later became a local official, she was mother to five children. It is believed Francesco Del Giocondo commissioned the portrait to celebrate either his wife's pregnancy or the purchase of a house around 1502 and 1503. After his death, Gherardini became a nun. She died in 1542 at the age of 63 and was said to be buried near the Sant'Orsola convent's altar. In 2014 scientists conducted a DNA test on bones fond at the convent which they believe belonged to Gherardini but the results are still to be released. Advertisement

'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.

To find out how da Vinci's illusions worked, the researchers set up test in which people either viewed the portraits from a distance or saw blurred versions, according to a report in Discover magazine.

The researchers used a series of experiments to examine how different points of view and levels of blur in the images themselves could alter a viewer’s perception.

The researchers asked volunteers to look at La Bella Principessa, Mona Lisa and another typical painting from the same era Portrait of a Girl, painted in 1470 by Piero del Pollaiuolo.

First they conducted a number of tests to see how a viewer’s distance from the portrait would influence their perception.

They found that when viewed from further away, both the Bella Principessa and Mona Lisa appeared to be smiling more than the portrait by del Pollaiuolo.

They also used digital manipulation to alter the level of blur in each of the paintings. They found as blur in the two da Vinci paintings was increased, the smiles appeared to increase.

The researchers asked volunteers to view digitally manipulated versions of three paintings - da Vinci's La Bella Principessa (top), Mona Lisa (middle) and Portrait of a Girl by Piero del Pollaiuolo (bottom). They changed the level of blur in the paintings to examine whether this changed the impression of a smile, as shown above

When viewed directly, the slant of the mouth in the Mona Lisa (pictured) is distinctly downwards. But as the eye moves elsewhere to examine other features, the mouth appears to take an upward turn, creating a smile that can only be seen indirectly

In del Pollaiuolo’s painting the perception of the girl’s smile remained broadly the same, and actually decreased slightly as the blur worsened.

This suggests da Vinci’s technique specifically relies upon the viewer seeing the mouth with unfocused eyes in order for the smile to appear.

In a final experiment, the researchers tested whether the mouth or the eyes were responsible for the mysterious smile illusion by masking the features with black rectangles.

They found when the mouth was obscured, the illusion did not appear to work, but when the eyes were obscured, viewers still detected the hint of contentment in La Bella Principessa’s smile.

Soranzo told Discover, 'Given da Vinci's mastery of the technique, and its subsequent use in the Mona Lisa, it is quite conceivable that the ambiguity of the effect was intentional.'

The researchers used masks on La Bella Principessa to examine whether it was just the mouth or the eyes as well that were involved in the illusion of her smile (as shown above). Their results suggest it is just the mouth that appears to be involved, despite claims by some that da Vinci made his paintings 'smile through their eyes'

La Bella Principessa is thought to depict 13-year-old Bianca Sforza, the daughter of Ludovico Sforza, duke of Milan, who was to be married to a commander of the duke's Milanese forces.

But, she would be dead within months of the marriage, having suffered a possible ectopic pregnancy, adding poignancy to her expression in her portrait.

He added that da Vinci may have first attempted the technique even earlier in his 1483 work 'Virgin of the Rocks.'

Michael Pickard, from the University of Sunderland co-authored a 2013 study by the same team.

'With his knowledge of the turbulence surrounding the Court of Milan at that time, Leonardo would have been aware of inner tensions between the fresh innocence of a young girl on the threshold of womanhood and her impending marriage and courtly destiny,' he said.

'It is also not difficult to believe that Leonardo would have seen below the surface and wanted to capture the subtle essence of the girl, using a technique he would so famously master in the Mona Lisa.'