66 MOVEMENT 



images, but it is difficult to ensure perfect regularity 

 of movement.* 



A better method of producing the same result 

 consists in reflecting the image of the object by 

 means of a revolving mirror. The image thus be- 

 comes deflected before it enters the camera. The 

 mirror, silvered by Foucault's method, rotates on a 

 vertical axis, and by means of clockwork it is easy 

 to ensure a uniform movement. Any speed that is 

 desired can be obtained. 



By these different ways of separating the image, 

 the range of chronophotography on fixed plates can 

 be considerably extended. For thus we are enabled 

 to record movements executed on the same spot, or 

 of extreme degrees of slowness. At the same time 

 the method ceases to be applicable when the duration 

 of the movement is greatly prolonged, when a large 

 number of images are required, or when the dimensions 

 of the plate will not contain the images. Neither is 

 it applicable when the moving object is dark and the 

 background light. Recourse must then be had to 

 a new method. This is chronophotography on a 

 moving plate ; it will be described further on. 



* Another inconvenience presented by this method is that there is 

 a risk of the apparatus itself introducing a source of error. For the 

 circular diaphragm rotating at a y:reat rate tends to preserve its own 

 plane of rotation, and consequently to become distorted when exposed 

 to sudden movement. 



