This is "A Female Figure resting on a Sword". Painted in the Italian north, possibly by a French artist working in Rome in the seventeenth century. The subject of this painting is probably Saint Catherine of Alexandria, who was tortured and beheaded in the fourth century.
Look at the way the figure's hands are clasped together - it's beautifully modelled with interlocking fingers. But if you compare that to the folds of her white undershirt, they're incredibly crisp. It's a great example of how the artist has used soft modelling alongside sharp, defined lines.
The background is quite simple, which makes the figure really stand out. You can almost feel the strong lighting drawing your eye to her face. This style of using light and dark to create depth is actually inspired by Caravaggio, who was a major influence on many artists working in Rome at this time.
This painting is particularly interesting because it's been found that there's another image underneath. The underlying layer shows a landscape with two figures - possibly the Virgin and Child. It looks like it might be from the late 15th or early 16th century, so we're talking about a really old foundation beneath this later work.
The artist hasn't been identified, but they clearly knew what they were doing when it came to handling the paint. The overall effect is one of great three-dimensionality - you feel like the figure could step out of the painting at any moment.
A Female Figure resting on a Sword hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.