Book excerpt: In Mecanisme de la Physionomie Humaine, the great nineteenth-century French neurologist Duchenne de Boulogne combined his intimate knowledge of facial anatomy with his skill in photography and expertise in using electricity to stimulate individual facial muscles to produce a fascinating interpretation of the ways in which the human face portrays emotions. This book was pivotal in the development of psychology and physiology as it marked the first time that photography had been used to illustrate, and therefore "prove," a series of experiments. Duchenne's book, which contained over 100 original photographic prints pasted into an accompanying Album, was rare, even when it first appeared in 1862. Duchenne was a superb clinical neurologist and in this study he applied his enormous experience in neurological research to the question of the mechanism of human facial expression. Duchenne has been little cited and little known in this century; his book has been virtually unobtainable, and copies are available in only a few libraries in the United States and Europe.