A camera and a computer can "see" something that perhaps millions of pairs of eyes failed to see for 480 years: the Mona Lisa's necklace. The discovery about the famous painting was made by an American physicist. The Mona Lisa, painted by the Italian, Leonardo da Vinci, has been on show for many years in Paris. Now a row of white spots on the neck have been "seen". They are said to be the remains   of a necklace which the artist later painted over. The painting will be examined further by using infrared scanning   techniques. Laser   technology can be used to show what lies directly under the surface of the finished work, too. This will help us to understand how the artist drew the picture: Did he begin with a quick-made drawing or have second thoughts as he painted the picture? This kind of knowledge will show us the famous artist at work. It may also help to explain Mona Lisa's smile, which has puzzled art lovers for centuries. According to the passage, the white spots on Mona Lisa's neck   _  .
are still invisible to visitors have turned into a necklace have already appeared before visitors are looking more and more like a necklace