bifocals vs pair of eyeglasses

bifocals

noun
  • Spectacles that have corrective lenses of two different powers; used by people who need both distance and reading glasses. 

pair of eyeglasses

noun
  • A pair of lenses set in a frame worn on the nose and ears in order to correct deficiencies in eyesight or to ornament the face. 

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