Silver iodide layered on a 3.25x4.25-inch chunk of glass tells of a time when men made money the old-fashioned way - with machines. A gold-embossed frame on this paper-edged magic lantern slide credits "William H. Rau, Photographer - Philadelphia, Penna. On the other side, written in fine ink, are the words "Penna. Phila. - New U.S. Mint Milling Room." The men perch on stools beside the iron flywheels of massive, belt-driven machines, holding as still as they can for Mr. Rau to close the shutter.