Everyone has seen holograms in charm bracelets, on credit cards, in science fairs, and on the cover of NATIONAL GEOGRAPHIC. A hologram can be understood as a moire pattern made with light waves. The hologram was inconceivable outside of a small cabal of theoretical physicists who plotted the mathematics since the beginning of this century, and a hologram couldn't be produced until the laser was invented to provide the necessary coherent waves. Many soakers must have made the discovery since bathing became intentional: Without any parade of mathematics, they discovered the hologram while soaking in their tub, with their big toe protruding above the surface of the steamy water. Ordinary light can be represented by splashing water in a bathtub until the surface is covered with random waves. In contrast, a slow drip from the faucet into a tub full of still water covers the surface with a regular, parallel wave pattern, illustrating the coherent light waves in a laser beam. Now, if you set a firm object in the tub so that it breaks the surface of the water, the regular waves will reflect from the surface of the object, and the reflected waves intersect the regular waves at an angle determined by the angle of reflection. Soon the entire surface of the water is covered with a pattern of regular waves perfusing with reflected waves. The remarkable feature of this pattern is that the angles of intersection are about the same over the entire surface; remember Euclid's theorem that a straight line intersects all parallel lines at the same angle. In other words, every part of the surface of the water contains the same information; you have created a hologram in your bathtub. If it were possible to reverse the flow of the reflected waves, they would return to their points of reflection to reconstruct the cross section of the object. The coherent light of a laser beam functions as a moire screen. Waves reflected from an object function as a second moire screen to generate an interference pattern, like you can see in your tub demonstration. A sheet of microfine-grain photographic film placed in the laser light will record a moire pattern of swirls and whorls; no camera or lens is needed. The extremely fine lines defining wave interference function as a diffraction grating and a diffraction grating functions as a lens. When another laser beam is shone through the film, the rays are bent to meet at focal points corresponding to resolved points on the surface of the photographed object so that projection reconstructs the form of the original solid object as a ghostly image in space viewable from all sides. A real ghost is a natural hologram. Any movement while a hologram is being exposed will blur one wave into another to register nothing but an even light fog. Therefore, holograms are photographed on heavy stone tables firmly set on bedrock --- after consulting timetables for nearby railways and jetliner takeoffs, and the children are asleep. Furthermore, the photographic film must be at least as fine-grained as the moire pattern in the laser light if the image is to be resolved. Film speed varies directly with graininess, so holographic microfilm is the slowest emulsion since Daguerre clamped his portrait sitters in a vise. The growing interest in holography is likely to stimulate Great Yellow Father to improve the speed of fine-grain film, while inventors will figure out ways to simulate holography by more practical techniques formaking television images in-the-round; the solution to cheap holograms seems to be in computer graphics.