450 



SCIENCE OF HOME AND COMMUNITY 



Throwing pictures on the screen. For showing pictures 

 on the screen, three essentials are needed: a projecting 

 apparatus, a source of strong light, and a screen or white 

 surface. This projecting apparatus is the same in principle 

 as the stereopticon, or magic lantern, the chief difference 

 being in the mechanism by which the film is passed through 

 the machine. 



Parts of a stereopticon. The essential parts of a stereop- 

 ticon are shown in Figure 179. These consist of a source of 



light A ; a set of 

 condensing lenses C, 

 which concentrate 

 the light upon the 

 lantern slide 5. This 

 is focused by the lens 

 L upon the screen S f . 

 This lens inverts 

 the image so that 

 the slide is placed 

 in the holder upside 

 down. 



FIG. 179. Stereopticon. 



Projecting apparatus. The delicate part of the moving- 

 picture lantern is the mechanism by which the film is passed 

 through the machine. This film must have an intermittent 

 motion, the same as in the camera, so as to stop for a fraction 

 of a second to allow light to pass through and throw a distinct 

 image on the screen. The film is mounted on a spool and 

 led over a toothed sprocket, which fits into the perforations. 

 A loop is then formed, and the film passes into the gate 

 behind the lens. As the shutter revolves, shutting off the 

 light from the picture, the film is given a jerk downwards, 

 thus bringing the next picture into the gate, in exactly the 

 same position as the previous one. The film is then con- 

 nected with another sprocket and finally rolled up on another 

 spool. The machine may be operated either by hand or by 



