Known also simply as a lamp clock, this fulfilled the dual purpose of providing light, like an ordinary oil lamp, and telling the time. The principle is simple: a conventional clock movement revolves a lampshade marked with the hours and quarters and read against a fixed pointer. Another version has a conventional dial and hands on one side of the opal or frosted-glass shade, illuminated by the flame of the lamp behind. The idea never seems to have been exploited widely; the form originated in the 19th century and several designs were produced in France. Another, earlier type of oil lamp clock, datable to the 18th century, consisted of a pewter stand, not unlike a candlestick, at the top of which a glass container of oil fed a wick supported by a holder projecting at right angles to the stand. The glass container was marked with hour divisions which corresponded to the time it took for the oil level to drop in the container as it burned at the wick.