Crawling on all fours, I made steadily but slowly towards them, till at last, raising my head to an aperture among the leaves, I could see clear down into a little green dell beside the marsh, and closely set about with trees, where Long John Silver and another of the crew stood face to face in conversation.
There are no more uses of "aperture" in the book.
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The higher the f-stop, the more closed the camera’s aperture.
It wandered along the crest of the hill and terminated before a fragment of wall pierced by a rough aperture which had once been a door.