120 



Popular Science Monthly 



Why Wheels Run Backward on the 

 Motion Picture Screen 



WHEN the moving-picture camera 

 takes a picture the film is not exposed 

 continuously. Instead it is uncovered and 

 then covered again in very rapid succession 

 by a black disk edged with a circle of 

 holes which are 

 swiftly rotated 

 across the front 

 of the lens. In 

 this way a series 

 of pictures is taken 

 which represents 

 the successive 

 movements of the 

 subject that is 

 photographed. 

 But because of 

 this very fact, the 

 speed at which an 

 object appears to 

 rotate when the 

 developed film is 

 projected on the 



case of this kind, if the spoke and the 

 camera keep turning at constant speeds, 

 when the spoke is projected on the screen 

 it will appear to be standing perfectly still. 

 Again, the wheels on fast moving cars 

 often appear as if they were moving 

 backward while the car is going for- 

 ward. This would 

 happen if the 

 spoke A , shown 

 in the bottom row 

 of the drawings, 

 had been revolv- 

 ing so fast when it 

 was photographed, 

 that during the 

 interval between 

 one exposure and 

 the next, the 

 wheel revolved all 

 the way around 

 from A to B. 

 When the next 



The speed of rotation of the wheels may be such as 

 to make it appear to stand still as in the top row, or 

 even to be running backward as in the bottom row 



screen is very seldom the speed at which it 

 actually rotated. 



Take, for instance, a four-spoked wheel, 

 such as shown in the illustration, where the 

 wheel rotates at a speed such that after one 

 picture of the film is exposed, the spoke 

 A has turned around just enough to show 

 A at the position of B when the next pic- 

 ture is exposed. It is evident that in a 



exposure was 

 made the spoke 

 appeared at C, and so on. When this film is 

 run off and projected on the screen, the 

 spoke will seem to run backward. No 

 matter how many spokes there are on 

 the wheel it is evident that the effect on 

 each one will be the same, and the 'entire 

 wheel will appear to turn backward at 

 the same speed as that of each spoke. It 

 is merely an optical illusion. 



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