The word firmament is commonly understood to mean sky. But, it derives from a Latin translation of a Hebrew word, raki’a, which means a great expanse, a possible description of both the dome of the sky and the floor of the earth. In Firmament, cyanometers, a nineteenth-century device for measuring the degree of blue in the sky, highlight this primordial connection between what is above and below as they are used to identify the colors of the sky and a selection of blue rocks.

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