70 



MOVEMENT 



them. These diaphragms rotate in opposite directions, 

 and as two foramina pass each other an exposure 

 occurs, and the plate is illuminated. By this arrange- 

 ment we can employ discs of small size, and conse- 

 quently greatly reduce the total dimensions of the 

 apparatus. In fact, the dimensions need not exceed 

 the size of an ordinary (24 X 30) camera. The shaft, 

 which determines the revolution of the diaphragms, 

 receives its own motion from wheels, which are worked 



Fig. 48. — "Park slide for negative. 



by a crank. At present, however, we have no space 

 for a detailed description. 



Now, in focussing, the position of the slide must 

 vary considerably, and the two parts of the apparatus 

 must be more or less separated from one another. 

 The shaft must therefore be able to accommodate 

 itself to these changes in distance, and for this reason 

 is composed of square tubes, which slide one within 

 the other with what is technically called a telescopic 

 action. 



Erection of a Dark Background for Chronophoto- 

 graphy. — The principles of chronophotography on 



