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"If a pair of concave mirrors be set facing each other, as shown in figure 7, and a source of heat be placed in the focus of one, a thermometer in the focus of the other will quickly show a rise in temperature though the mirrors are many feet apart." Louis Derr, 1911

A Pair of Concave Mirrors Set Facing Each Other

"If a pair of concave mirrors be set facing each other, as shown in figure 7, and a source of heat be…