24 ZOOPEAXOGRAPHT 



side by side, and the exposors were each connected with 

 the exposing motor by separate lengths of a hundred feet 

 of cable. ^ The two series of cameras were pointed to a 

 rapidly revolving disc of five feet diameter. The surface 

 of the disc was black, with narrow white lines radiating 

 from the center to the edge like the spokes of a wheel . A 

 microscopic examination of the two series of resulting 

 negatives proved that no variation could be discovered 

 in the sychronous action of ten of the duplicated series 

 of exposures, and that in the remaining two a variation 

 existed in the simultaneity of a few ten-thousandths of 

 a second — a result sufficiently near to simultaneity 

 for all ordinary photographic work. 



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Fig. 9. 



A reproduction of the chronographic record of one 

 of these experiments is seen in figure 9. 



The first line records the revolution of the disc; the 

 second the vibration of the tuning fork; and each 

 group of three long double markings in the third line 

 indicates a photographic exposure. 



The shortest exposures made at the University 

 were — approximately — the one six-thousandth part 

 of a second; such brief exposures are hoAvever for this 

 class of investigation very rarely needed. 



Some horses galloping at full speed will, for a 

 short distance, cover about fifty-six or fifty-eight feet 



