New Developments in Science 585 



graph which operates a loud speaker behind the screen. In this 

 form of talking picture, the phonograph and the motion picture 

 machine must be carefully adjusted so that each sound comes at 

 exactly the right point in the picture. 



The second form of talking picture is now much more widely 

 used. In this type, the pictures and sounds are both recorded on 

 the same film. The pictures are taken just as in silent motion 

 pictures and the sounds are recorded on the film at the side of 

 the pictures. As the film is run through the camera, a beam of 

 light from a lamp passes through a narrow slit and falls upon this 

 sound strip. A special shutter, operated by the sound waves of 

 the actors' voices, varies the width of the slit through which the 

 light passes, and thus regulates the amount of light which strikes 

 the sound strip. When the film is developed, the sound strip 

 appears darker in some places than others, according to the 

 amount of light that struck it as it passed, the slit. 



When a film is shown, the sound strip passes between a lamp 

 and a photoelectric cell. The varying darkness and lightness of 

 the sound strip causes the beam of light passing through the film 

 to the photoelectric cell to vary in brightness. This causes cor- 

 responding variations in the strength of the electric current which 

 passes through the photoelectric cell and which operates the loud 

 speakers behind the screen. In this way, the loud speakers are 

 made to give out the same sounds that were recorded on the 

 sound strip of the film when the moving picture was taken. 



The next time you go to see a motion picture, stop and think 

 of the wonderful way in which the sounds and the actors' voices 

 are preserved and reproduced for your enjoyment. 



Telephotography. A civil war broke out in Austria. In a 

 great apartment building in Vienna a group of armed revolu- 

 tionists tried to defend themselves against the government sol- 

 diers. Bullets from rifles and machine guns crashed through the 

 windows. Explosive shells ripped holes in the walls. The revolu- 

 tionists were forced to surrender. 



A few hours after the fighting was over, a New York paper 

 printed a picture of the wrecked apartment house. How had that 

 picture traveled three thousand miles so quickly? By boat and 



