From Zelda Wiki.org
In Link's Awakening, the Magnifying Lens is the last item in the Trade Sequence. It can be found by inserting the scale from Martha into her statue in Martha's Bay. The Magnifiying Lens can be used to find things not normally visible to the human eye (making it quite similar in function to the Lens of Truth). Using it allows Link to encounter a creature in Toronbo Shores with whom he can trade for a boomerang. It allows Link to read the book found in the Library in Mabe Village with the fine print that contains the answer to the puzzle in the Wind Fish's Egg.
