There's always a blacker black and a whiter white. I followed a
   book once that was supposed to train me to "see without
   glasses". Ultimately, likely, mostly a crock but still some
   useful exercises in there. One of them I liked: It's to improve
   contrast-ability in your brain so that your vision has a more
   'contrasty' place to go: You close your eyes and imagine the
   blackest black you can imagine. Go as black as you can. Hold it
   there. Then, go one shade blacker.