Double 8 movie film --- The first successful motion picture system was an invention of Thomas Edison. His system used 35mm wide long rolls of film. To transport the film through the camera smoothly, the film was punched on the left and right edges with holes (sprocket holes) that engaged with gears in the film transport mechanism.
Movies and 35mm wide film became popular. Movie cameras that accepted 35mm long rolls were big and cumbersome. Kodak and others soon introduced smaller systems using 16mm wide film.
Next, smaller cameras were designed especially for amateur movie photographers (home movies). These systems (you have one) accepted 16mm wide rolls of film. The film is rolled up on a flanged spool (reel) that accepted a 25 foot length of film.
The camera is supplied with an empty film reel. To load, this empty reel is moved to the take-up side of the camera. In subdued light, a new roll of 16mm film is unwrapped and placed in the receiving supply side of the camera. This special film featured a jet black backing called a “rem-ject”. This opaque backing protects the underlying film in the rolls from accidental exposure to light. The photographer threads the film from the supply roll to the take-up roll following the film path, which is specified by arrows engraved inside of the camera.
Once the new roll of film is threaded, the back is closed. Now the photographer begins to shoot movies. In just about 5 minutes of operation, the 25 foot long roll will be exposed. The entire 25 feet of film has now been re-spooled on what was an empty take-up reel.
Now the photographer must pause, open the camera back, remove the take-up reel with its exposed film, turn the reel over, and reinsert this reel in the supply positon. Now the photographer threads this film again. Once the camera is loaded for the second time, the photographer continues shooting for about 5 minutes.
Now the camera only exposes ½ of 16mm width on the first pass. The second pass exposes the other ½ of the width of the film. After the two passes through the camera, the film is sent to a photofinishing lab for developing. At the lab, the film is developed.
After the film is developed, the lab places the 25 foot long roll into a slitting machine. This machine slits the roll down the middle making two rolls of 8mm wide -- each 25 feet in length. The two halves are spliced together making a finished roll of movie film 50 feet long. At home the family enjoys about 10 minutes of home moves.
Because the loaded film is actually 16mm wide, this system is called “double 8”