The ancient Chinese, Greeks, Incas, and Romans—and no doubt others—discovered very early on that curved mirrors could concentrate the sun's rays onto anything burnable with enough intensity to cause the object to burst into flames in seconds. Because of these instruments' ability to start fires, the ancients, no matter their language, almost universally referred to them as burning mirrors. Both the secular and religious worlds took advantage of these devices to ignite fires, because, as Pliny observed, they "set things alight more easily" than any other way known at the time.
In the ancient Chinese kitchen, for example, a burning mirror was as common as a pot or pan. On sunny days, the son in charge of lighting the family stove took the concave mirror outside and concentrated the sun's rays onto kindling. As soon as the family stove fired up, the women could cook.
In religious rites, those in charge preferred to ignite sacred flames with burning mirrors. The rays of the sun seemed a more spiritual way of creating fire than human hands. Plutarch tells us that when the sacred fires in Greece and Rome were accidently extinguished (for instance, when the sacred lamp was put out at Athens, the temple burned at Delphi by the Medes, or the altar overturned and the fire extinguished in Rome), it was deemed an impiety to re-light the flames from other fires. New fires had to be lit by "drawing the pure and unpolluted flame from sunbeams." These new flames were generally kindled with concave brass mirrors.
Far across the seas, the Incas of Peru similarly ignited their holy fires with solar energy. Believing themselves the Children of the Sun, they celebrated the summer solstice with great solemnity. Temple virgins concluded the ceremony by flourishing their concave silver mirrors, mounted on gold, gem-encrusted frames. They focused the sun's rays onto cotton wool, which burst into flame. As the solstitial flame had to be lit on that day and no other, if the sun didn't shine the virgins made fire the secular way, rubbing two sticks against each other. But they shook in fear on such occasions, taking it as an evil omen, for, in the words of J. Frazer (The Golden Bough), "they said the Sun must be angry with them, since He refused to kindle the flame with His own hand."
The Incas, Chinese, and Greeks believed their burning mirrors "collected" fire from the sun. Theophratus, one of the first great naturalists, writing in the 4th century B.C.E., acknowledged that many of his contemporaries "believe they are catching sun rays when making a fire." With the common use of burning mirrors, a good number of people in the ancient world concluded that the flames of the fusion-reactor sun, 93 million miles away, and those chemical, atom-conserving flames here on Earth were one and the same. Explaining the conversion of visible light to flame (and distinguishing chemical from nuclear fire) would take much more than another millennium. But linking solar radiance to warmth, heat, cooking, light, and reverence was joyfully intuitive.