10 THE MECHANISM OF 



and skill of one of his principal assistants, Mr. Lino F. Rondi- 

 nella, and it is to him that the writer is indebted for the descrip- 

 tions, drawings, and diagrams of the apparatus used. Most of 

 the work done in instantaneous photography has been of an un- 

 systematic and desultory nature and limited scope. In this work, 

 which still leaves much to be done, we have the first systematic 

 attempt to record the motions of a variety of animals. 



Professor Thomas Eakins, director of the Pennsylvania Academy 

 of the Fine Arts, undertook, under the auspices of the University 

 of Pennsylvania, an investigation of some mechanisms of animal 

 movement, in which he is much interested. His apparatus will 

 be of great interest. He chose as his principal means the photo- 

 graphic method of Professor Marey, of France. The object to 

 be studied is moved in sunlight before a dark background, pre- 

 ferably a deep recess painted black. A photographic camera 

 is set up. A disk with one or more openings in it is rapidly 

 revolved in front of the lens. While the opening is passing 

 the lens the moving object is photographed ; then darkness in 

 the camera until an opening again arrives. The moving object 

 has now a new position, and is photographed anew on the same 

 plate, and so on again and again as often as required, or until 

 the object has moved beyond the range of the lens or plate. On 

 the same plate is then developed a series of images easily com- 

 pared. With the present sensitiveness of the gelatin dry plate 

 a tolerably good image may be had of any light object moving in 

 the sunlight not too fast. If the object moves too fast, the speed 

 at which the opening passes the lens must be increased, or the 

 opening itself narrowed, either of which diminishes the amount 

 of available light, and the image becomes weak. 



So in adjusting the instrument for a contemplated fast move- 

 ment one must choose somewhere between an image weak but 

 sharp and an image strong, but blurred from the movement. 



A brief description of the whole apparatus as used by Mr. 

 Eakins may be interesting to the physiologists who may wish to 

 avail themselves of this modern instrument of research. 



Instead of one large disk, as in Professor Marey's apparatus, he 

 used two small ones on the same arbor, but geared at different rates 

 of speed, — namely, one to eight. 



If a clock-face should have disks instead of hands, and the 



