The physical technique of producing a photographic image of an opaque specimen by transmitting a beam of X-rays through it onto an adjacent photographic film; the image results from variations in thickness, density, and chemical composition of the specimen. The technique is used to study details of structure, decoration, or composition invisible to the naked eye. X-rays are normally used, though gamma-rays may be employed in circumstances where an X-ray tube is difficult to manipulate. Differential absorption of the rays denotes variations in composition, and these are shown on the film as contrasting light and dark tones. This form of examination works well on iron objects where rust and corrosion products prevent proper study of the object. Radiography is also used in the study of biological material, such as bone and mummies. Radiography has also been used to examine sedimentary structures. Microradiographic techniques have been developed to examine the atomic structure of crystalline substances.