masterwork vs opuscule

masterwork

noun
  • A piece done to prove possession of skill sufficient to be ranked a master. 

  • A piece of quality, indicative of having been made by a master; a masterpiece. 

opuscule

noun
  • A small or petty work. 

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