14 June 2013 Last updated at 11:46 ET
Italian composer Luigi Cherubini wrote Medee in 1797 - but blacked out the final aria on the original score.
Beethoven was said to have regarded Cherubini as his greatest contemporary.
Now scientists from the University of Manchester and Stanford University have used X-ray equipment to "see" under the blacked-out part to the notes below.
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