Magnifying Lens

From Zelda Wiki.org

In Link's Awakening, the Magnifying Lens is the last item in the Trade Sequence. It was what Link managed to find when he inserted the scale from Martha into her statue in Martha's Bay. The Magnifiying Lens was used by Link to find things that weren't normally visible to the human eye (making it quite similar in function to the Lens of Truth). This gave him access to a creature in Toronbo Shores that he could trade an item with for a boomerang. It also let him see the book in the Library in Mabe Village with the fine print, that contained the answer to the puzzle in the Wind Fish's Egg.