She is widely seen as proof that good looks can last forever. But, at nearly 500 years of age, _  The health of the famous picture, painted by Leonardo da Vinci in 1505, is getting worse by the year, according to the Louvre museum where it is housed. "The thin, wooden panel on which the Mona Lisa is painted in oil has changed shape since experts checked it two years ago," the museum said. Visitors have noticed the changes but repairing the world's most famous painting is not easy. Experts are not sure about the materials the Italian artist used and their current chemical state. Nearly 6 million people go to see the Mona Lisa every year, many attracted by the mystery of her smile. "It is very interesting that when you're not looking at her, she seems to be smiling, and then you look at her and she stops," said Professor Margaret Livingstone of Harvard University. "It's because direct vision is excellent at picking up detail, but less suited to looking at shadows. Da Vinci painted the smile in shadows." However, the actual history of the Mona Lisa is just as mysterious as the smile. Da Vinci himself loved it so much that he always carried it with him, until it was eventually sold to France's King Francis I in 1519. In 1911, the painting was stolen from the Louvre by a former employee, who took it out of the museum hidden under his coat. He said he planned to return it to Italy. The painting was sent back to France two years later. During World War II, French hid the painting in small towns to keep it out of the hands of German forces. Like many old ladies, the Mona Lisa has some interesting stories to tell. The smile of the Mona Lisa can only be seen  _  .
by indirect vision at a distance by direct vision in shadows