Fig. 1. Photograph of the wagon with which the experiments were made. 
plates a series of points from each signal is formed. The distance in 
space that each signal has moved between two exposures can thus be 
measured. The velocity is determined according to the formula. 
r= =2 (1) 
For the determination of the error in this formula test has shown 
that one can regard At as a constant. Thus the error originates from 
the determination of As. The measuring instrument was a Zeiss stereo- 
comparator. With 2,8 m photography distance the standard error of 
As is about 0,5 mm. Thus the standard error of the velocity is: 
V 
m m (2) 
U = As As 
To control the geometric conditions of the camera a straight line 
marked with 20 cm intervals was placed in the background. Thus one 
of the most serious errors (g-error or vergence error) can be controlled. 
See Hallert [2]. 
To distinguish different signals a constant light of less intensity than 
the stroboscope flash can be used together with the stroboscope lamp. 
3