Step 1: The question uses a simile from the poem "Martha" by Walter de la Mare.
Step 2: The poem describes the captivating nature of Martha as she tells her stories. Her physical features reflect the magical quality of her tales.
Step 3: The poem uses figurative language to describe her. Her eyes, which are a window to her imaginative world, are described as being as calm and peaceful ('tranquil') as 'dreams'. This simile connects her physical appearance directly to the fantastical nature of her storytelling.