invention vs opus

invention

noun
  • The act of inventing. 

  • Something invented. 

  • The capacity to invent. 

  • A small, self-contained composition, particularly those in J.S. Bach’s Two- and Three-part Inventions. 

opus

noun
  • A work, especially of art. 

  • A work of music or set of works with a specified rank in an ordering of a composer's complete published works. 

How often have the words invention and opus occurred in a corpus of books? (source: Google Ngram Viewer )