"It is also highly probable that these photographic investigations--which were executed with wet collodion plates, with exposures not exceeding in some instances the one five-thousandth part of a second--will dispel many popular illusions as to the gait of a horse, and that future and more exhaustive experiments, with the advantages of recent chemical discoveries, will completely unveil to the artist all the visible muscular action of men and animals during their most rapid movements.... "The employment of automatic apparatus for the purpose of obtaining a regulated succession of photographic exposures is too recent for its value to be properly understood, or to be generally used for scientific experiment. At some future time the explorer for hidden truths will find it indispensable for his investigations." In 1883, the University of Pennsylvania, with an enlightened exercise of its functions as a contributor to human knowledge, instructed the author to make, under its auspices, a comprehensive investigation of "Animal Locomotion" in the broadest significance of the words. A DIAGRAM OF THE STUDIO and the arrangement of the apparatus used for this purpose is here given. [Illustration] TT represents the track along which the model M was caused to move. B is the background, divided into spaces of 5 centimetres square for the purpose of measurement. L, a horizontal battery of electro-photographic cameras, parallel to the line of motion (at a distance of 15 metres or about 48 feet therefrom), for a series of 12 lateral exposures. R, a vertical battery of electro-photographic cameras, at right angles to the lateral battery, for a series of 12 _rear_ foreshortenings. F, a horizontal battery of electro-photographic cameras, at any suitable angle to the lateral battery for a series of _front_ foreshortenings. O, the position of the electric batteries, a chronograph for recording the time intervals of exposures, and other apparatus used in the investigation. A clock-work apparatus, set in motion at the will of the operator, distributed a series of electric currents, and synchronously effected consecutive exposures in each of the three batteries of cameras. The intervals of exposures were recorded by the chronograph, and divided into thousandths of a second. These intervals could be varied at will from seventeen one-thousandth parts of a second to several seconds. The task of making the original negatives was completed in 1885; the remaining years have been devoted to the preparation of the work for publication. [Illustration: LATERAL elevation of some consecutive phases of action by representative horses. Each line illustrates the successive fallings of the feet during a single stride. After the last phase illustrated, the feet, during continuous motion, will revert practically to their position in the first phase.