TIME 



17 



images succeed one another at absolutely regular 

 intervals, because both the movement of the needle 

 on the dial and that of the circular diaphragms are 

 equally uniform. 



Measurement of the Intervals of Time which separate 

 Successive Exposures. — By reason of the clear definition 

 of the images, they can be accurately measured, not 

 by the time of exposure, which is too short to be 

 appreciated, but by the in- 

 tervals of time between suc- 

 cessive exposures. Now, this 

 is the important point in the 

 measurements which we shall 

 have to make of the duration 

 of certain phenomena. 



Provided that one can ar- 

 range a reliable clockwork 

 mechanism so as to move the 

 needle round the dial at a 

 uniform rate, it does not mat- 

 ter what rate of movement 



is imparted to the circular diaphragms, the interval 

 between two exposures can always be measured by 

 the angle contained between two consecutive images 

 of the needle. If, during the time, an object, visible 

 in the field of the lens, happens to move, there will 

 be found, on the sensitized plate, several of its images 

 in various positions and at various distances from one 

 another. For the purpose of measuring the intervals 

 of time between such successive positions of the object, 

 the changes in position of the needle on the chrono- 

 graphic dial will serve as an index. 



The further applications of this kind of time 

 measurement will be seen when we come to discuss 

 Chronophotography. 



Fig. 12.— Successive positions of 

 the needle on the chronomeiric 

 dial, measuring the intervals of 

 time separating the successive 

 exposures. 



