20 MOVEMENT 



it takes.* It must be possible, however, to fasten this 

 point directly or indirectly to the needle which is to 

 trace the trajectory, and, further, the propelling force 

 must be sufficient to work the mechanism of the 

 recording instrument, and that, too, without modifica- 

 tion of the movement. But if the point is inaccessible, 

 if the propelling force is too feeble, or if it follows 

 a very complicated course, we must introduce new 

 conditions, and employ photography in the special 

 manner we are about to describe. 



Principles of Photography with a Dark Background. — 

 When a camera faces a dark background, no impression 

 is made on the sensitized plate, because no light 

 reaches it; but if a very luminous object is placed 

 between the background and the lens, light will be 

 reflected and an image imprinted on the plate. If, 

 while the object-glass is uncovered, the white object 

 changes its position, there will be reproduced on the 

 plate a track which exactly corresponds to the move- 

 ments of the object. This, is the trajectory of the 

 object, or, to put it more precisely, the projection of 

 its trajectory on the surface of the sensitized plate. 

 The image will be more or less reduced in size 

 according to the distance of the object, and according 

 to the focal length of the objective. 



Photographic Trajectory of the Movements of a Point 

 in Space. — To demonstrate the advantages offered by 

 photography as a means of recording the trajectory of 

 a moving object, we will choose as an example a case 

 in which direct observation will afford us no informa- 

 tion, and in which a mechanical method of recording 

 will be impracticable. Suppose, for example, that 

 we wish to ascertain the various positions in space 



* The different proceedings for mechanically recording the move- 

 ments of a point in one or more directions in space have been given in 

 " The Graphic Method." 



