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The Mermaid's Scale is the final item in the Link's Awakening Trading Sequence before the reward. After Link returns the Necklace to its rightful owner, Martha the Mermaid, she will allow him to take a scale from her tail. If Link speaks to Martha afterwards, she will say that when she posed for Schule Donavitch, he had also wanted a scale.

Link can place the scale on the Mermaid Statue at Martha's Bay, which will then slide sideways, revealing the entrance to the cave which holds the Magnifying Lens.

Trivia

 * If Link presents the scale to Schule Donavitch, Schule will ask Link to complete the Mermaid Statue on his own, as he is busy painting a portrait of the Hippo Model.