The Dark Sign—Its Duration—Origins—Summer Solstice—Hercules and the Crab—Characteristics—Goddess Esmeralda and the Ancient Peruvians—The Emerald Isle—Moonstones—Blue Moon—Pearls—Cat's Eyes—Rock Crystals—Crystal-gazing—Virtues of the Crystal. Cancer, the fourth sign of the Zodiac, is situated in the northern celestial hemisphere; being composed of small stars, the brightest of which are only of the third magnitude, and very few visible to the naked eye, it was known in olden times as the dark…