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"lurid" means "gruesome, horrifying, or revolting," according to dictionary.com. The page means that people had "dark", or evil, parts of themselves that they ordinarily kept hidden away (dark pageway of the interior), but that the scarlet letter (the letter "A" she was forced to wear because she was accused of adultery) forced everyone to look at (illuminated by its gleam) the fact that they had such negative elements as a part of themselves.