specter vs will o' the wisp

specter

noun
  • A threatening mental image. 

  • Any of certain species of dragonfly of the genus Boyeria, family Aeshnidae. 

  • A ghostly apparition, a phantom. 

will o' the wisp

noun
  • A delusionary or otherwise unachievable goal that one feels compelled to pursue. 

  • A flickering light or lantern, especially over bogs, swamps or marshes, held by a supernatural creature who uses it to lure travelers at night to their demise. 

How often have the words specter and will o' the wisp occurred in a corpus of books? (source: Google Ngram Viewer )