Some wonderfully creative uses of X-rays have been in the world of art.But the most impressive one is to uncover what has been covered up.Details unfolded by an X-ray of a painting can ' t be seen any other way.For example, in the past, paintings were often repaired if small pieces of paint fell off the canvas  . These repairs cannot be seen just by looking at the painting.It takes an X-ray to discover where the repairs have been made. An X-ray of a painting is one of the best ways to tell if it was created by a famous painter or if it is a forgery .If there is a question as to whether an old master has painted a certain painting or not, X-rays are made and compared to those of paintings that are known to be original.If the brushstrokes   , for example, are shown to be totally different than in other work done by a certain artist, then the painting is proved to be a fake copy. Since modern paints are usually made from different materials than paints of hundreds of years ago, the image they leave on an X-ray film looks different.This is another way X-rays can be used to spot a forgery.A modem artist will try to make a forgery look hundreds of years old by painting dirty varnish on it or by using artificial means to get the varnish to look cracked.To the naked eye the forgery may look old, but when a careful study of the X-ray image is made, the _ is obvious. It was common many years ago for an artist to finish a painting, decide it wasn't any good, and paint a completely different picture on top of it.Or artists would paint over others' work --it was their version of recycling.Taking an X-ray is the only way to find out if there is a picture underneath the picture you see with your eyes.Sometimes museums will be more interested in the painting beneath than the one on top, and will restore the older one by having the top layer painstakingly removed. According to the passage,X-rays are used to   _  .
make repairs on paintings in the past identify the authenticity of an artwork change the materials of a forgery remove the recyclable paints