As is well known, radiography provides a means for non-destructive analysis in many fields outside of medicine and medical research. Perhaps the industrial applications have received the most publicity, but x-rays have been similarly employed in such fields as archaeology, paleontology, geology, biology, conchology, zoology, and entomology. One of the best known non-medical and non-industrial uses of radiography is in art museums, where paintings are examined for analysis of the paint, canvas, paper, or wood base. Quite recently similar investigations have been made in the field of philately, or stamp collecting. A postage stamp consists basically of a design printed on a small piece of thin paper. It is important to the collector to be able to differentiate various characteristics of stamps. These include: (1) the method of printing, such as intaglio, letterpress, offset, photogravure, or other process; (2) the type of paper—woven, laid, ribbed, pelure, etc.; (3) watermark, if any, in the paper. Such inf...