THE STROBOSCOPIC EFFECT. 
L. E. DODD. 
MATHEMATICS OF STROBOSCOPY. 
I. The Characteristic Stroboscopic Equation. 
Consider a straight row of equally spaced white dots on a 
black belt running at a constant velocity v over two pulleys. 
Let the dot spacing be D 0 . Suppose that the belt is in a 
darkened room so that even when the belt is at rest the dots 
are invisible. Let the moving belt be periodically illuminated 
By using two sets of removable discs, A and B, independent values of n 
and m can be used independent of any angular velocities. With the gear 
system not in action the angular velocities of discs A and B are equal, and 
the stationary stroboscopic condition results. With gear system in action 
the clutch is released, and by means of the adjustable belt system Vs may 
be given positive or negative values. Each disc of a set is provided with a 
circular row of equally spaced apertures concentric with the disc axis, but 
the* angular spacing is different for different discs. To avoid phase adjust- 
ments of discs A and B the present arrangement for periodic illumination 
was chosen in preference to a method where discs A and B are of the same 
size and the illuminations are by coincidences of similar circular rows of 
apertures with parallel light as illuminant. The instrument may be found 
useful to illustrate the building up of the various types of stroboscopic 
images, since lowering the angular velocity of the system does not affect the 
essential stroboscopic relations. Also, the part played in stroboscopy by 
retinal lag may be made clear. The stroboscopic effect given by the instru- 
ment may be viewed by observation of the front face of disc A (looking 
from the left in the figure), or the effect may be projected to a screen. 
