An estimate having been made of the interval of time which will be required, between each photographic exposure, to illustrate the complete movement, or that portion of the complete movement desired, the apparatus is adjusted to complete a succession of electric circuits at each required interval of time, and the motor is set in operation. When the series is to illustrate _progressive_ motion; upon the arrival of the model at the point marked "1" on the track, the operator, by pressing a button, completes an electric circuit, which immediately throws into gearing a portion of the apparatus hitherto at rest. By means of suitably arranged connections, an electric current is transmitted to each of the 3 cameras marked "1" in the various batteries, and an exposure is simultaneously made on each of the photographic plates, respectively, contained therein. At the end of the predetermined interval of time, a similar current is transmitted to each of the cameras marked "2," and another exposure made on each of the 3 next plates, and so forth until each series of exposures in each of the three batteries is completed. Assuming the operator to have exercised good judgment in regulating the speed of the apparatus, and in making the first electric contact at the proper time, and that the figures 1 to 12 represent the distance traversed by the model in executing the movement desired, the first three photographic exposures--that is, one exposure in each battery--will have been synchronously made when the model was passing the position marked "1" on the track T; the second three exposures will have been made when the model was passing the position marked "2," and so on until twelve successive exposures were simultaneously made in each of the three batteries. This perfect uniformity of time, speed, and distance, however, was not always obtained. When this monograph was commenced it was not intended by the author to give any more than a general idea of the method adopted for obtaining the results of his investigation; it has, however, been considered that a few illustrations and brief description of the apparatus devised and used by him may not be without interest to other students. For the use of these illustrations he is indebted to the courtesy of Rev. Jesse Y. Burk, the Secretary of the University, and to J. B. Lippincott Company, the publishers of "The Muybridge Work at the University of Pennsylvania," a book which contains, among other essays upon the subject, "Materials for a Memoir on Animal Locomotion, by Harrison Allen, M. D.," and "A Study of Some Normal and Abnormal Movements, by Francis X. Dercum, M.D., Ph.D." Figure 1 is a view of the building containing the lateral battery of twenty-four photographic cameras, all of which were used when as many consecutive phases of an act of motion were required. [Illustration: Fig. 1.]