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The halo or circle is thus formed. A thick and dark air intervening between the moon or any other star and our eye, by which means our sight is dilated and reflected, when now our sight is incident upon the outward circumference of the orb of that star, there presently seems a circle to appear. This circle thus appearing is called the ἅλως or _halo_; and there is constantly such a circle seen by us, when such a density of sight happens.