in prison vs under glass

in prison

prep
  • imprisoned; in a prison as a convict 

under glass

prep
  • In jail. 

  • In a greenhouse, a cold frame, or a similar structure; said of the propagation and growth of plants. 

  • In a glass case, like an item in a museum. 

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