452 SCIENCE OF HOME AND COMMUNITY 



and this gives the appearance of motion. Moving pictures 

 are really an illusion made possible through this peculiarity 

 of the eye. 



A picture is thrown on the screen and remains visible for 

 about one thirty-second of a second; then the screen is 

 darkened for about the same length of time by the passage 

 of the shutter, which cuts off the light. Thus the pictures 

 follow one another, one thirty-second of a second on the 

 screen and then one thirty-second of a second darkened. 

 As the image of the picture stays in the brain one twenty- 

 fourth of a second after the picture has left the screen, the 

 brain retains this impression during the one thirty-second 

 part of a second that the shutter cuts off the light. The 

 image of the second picture is thus impressed on the brain 

 before the first leaves it, so that the brain does not see the 

 darkened screen at all. If the pictures were run through 

 slowly, say at the rate of only five or six a second, then we 

 could distinguish between the pictures and the darkened 

 screen. 



LABORATORY EXERCISE 35 



Purpose. To illustrate why we seem to see a continuous set 

 of pictures at the movies. 



(The following exercise is taken from Rowell's Introduction to 

 General Science.} 



Directions. Obtain a piece of cardboard about three inches 

 square. Punch a hole in each of two opposite ends. Pass 

 through each a piece of string about fifteen inches long and tie 

 the ends together, making a loop. On one side draw a man's head 

 in colors or paste on it the picture of a man. On the other side 

 draw straight vertical lines. Put a loop over each thumb and 

 turn the cardboard till it winds up the strings. To put the 

 " man in prison " pull the thumbs apart thus giving a rapid 

 rotary motion to the cardboard. The man will seem to be 

 behind prison bars. Explain the result. How does this prin- 

 ciple hold in the movies? 



