The electric spark furnishes a most important means of taking instantaneous photographs, but the operator must perform in the dark. An electric spark can be obtained which lasts only the one two-thousandth of a second, and by its use as the sole illuminating agent we can get a photograph of a phase of movement lasting only that excessively short space of time, or, if we please, a succession of such phases by using a succession of sparks. Thus, a rifle bullet is readily photographed while in flight with scarcely perceptible distortion. A wheel revolving many hundred times a second can thus be photographed, and appears to be stationary. Dr. Schillings has applied this method to the photography of wild animals by night in the forests of tropical Africa, and has published an interesting book giving his photographic results. In order to take these pictures the track followed by certain animals has to be detected, and then a thread is stretched "breast-high" across the track, so that the animal coming along it by night shall pull the thread. Immediately the thread is pulled it sets an electric contact in action. There is a brief flash of one two-thousandth of a second, and a picture is taken by a camera previously fixed, out of harm's way, so as to focus the area where the thread was stretched. Dr. Schillings obtained some very remarkable photographs of "the night life of the forest" in this way--lions and leopards advancing on their prey were suddenly revealed, and the helpless antelope or other victim was shown crouching in the dark, or making a desperate effort to escape. The electric-spark method was applied by a friend of mine to demonstrate the movements by which a kitten falling backwards from a table succeeds in turning itself so as to alight on its feet. During a fall of less than 3 feet he obtained five successive spark-pictures of the kitten, which, I beg it may be clearly understood, was a pet kitten, and was neither frightened nor hurt by the proceedings. Instantaneous photographs, whether obtained by the use of an electric spark as a means of illumination, or by the less rapid method of a spring shutter working in combination with a sensitive film, which is jerked along so as to be exposed when the shutter is open and travel when it is shut, has been applied to the analysis of other movements than those I have mentioned, and has yet to be applied to many more, such as the crawling of insects and millipedes, and the beautiful rippling movement of the legs and body by which many marine worms swim. It has been extensively used in the study of human locomotion, and of the successive poses of the arms and legs in various athletic exercises, and in such games as baseball and golf.