seeing glass vs speculum

seeing glass

speculum

noun
  • A lookout place. 

  • A bright, lustrous patch of colour found on the wings of ducks and some other birds, usually situated on the distal portions of the secondary quills, and much more brilliant in the adult male than in the female. 

  • A mirror, especially one used in a telescope. 

  • A medical instrument used during an examination to dilate an orifice. 

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