This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The Projector The film positives are projected on a screen with the same kind of a lantern as is used for lantern slides, with the addition of the device for stepping the film through, one picture at a time, and flashing light on each picture as it remains stationary for an instant. The projector (Fig. 8) is composed of a lamp house, a condensing lens to make the beam of light converge upon the film for illuminating it evenly, a film-stepping device, and a projecting lens for throwing the enlarged picture of the illuminated film upon a screen.
Ill: Fig. 8 - Projector Complete
The lamp house is made of ordinary stovepipe metal and the dimensions given in the sketch are for a size suitable to use an acetylene or gas burner. The metal is laid out as shown by the pattern (Fig. 9) and bent on the dotted lines to form the sides and ends of the house. The joint may be riveted, or, if taken to a tinshop, lock-seamed. The cover is cut out as shown, the sides and ends having bent holes which are covered on the inside with perforated sheet metal, A. In order to deflect the light, a small angular strip, B, is riveted on so that its upper portion will cover the holes and allow a space for the heat to pass out. The cover may be hinged or set on like a cover on a can. The lamp house is attached to a sliding wood base for adjusting its position on the baseboard.
The condensing lenses are fixed into a metal barrel having a tapering end. This can be made of the same material as used in the lamp house. The parts can be rolled and a lock joint made at a local tinshop, or the pieces shaped over a wood form and riveted. Small L-shaped pieces are riveted to the inner surfaces to hold each lens in place. A rim is turned up on the back end of the metal tube for attaching the lens barrel to the lamp house.
Ill: Fig. 9 - Details Of The Lamp House
Ill: Fig. 10 - Details of the Lamp, Stepping Device and Base
An ordinary mantle or acetylene burner is attached to a gas pipe that has for its base a drop elbow fastened to a sliding board similar to the slide of the lamp house on the baseboard. A good reflector should be attached to a standard just back of the burner. The standard is also fastened to the sliding board. The proper distance of the light from the condensing lens can be easily set by this adjusting device. This arrangement is shown in Fig. 10 in the diagram entitled "lamp parts."
The device for stepping the film is a duplicate of the one used in the camera as described in Part I, with the exception of the lens. The lens should be about 2 in. in diameter with such a focal length that will give a picture of the required size, or a lens of 12-in. focus enlarging a 1-in. film to about 6 ft. at a distance of 24 ft. A regular lens fitted in a metal tube can be purchased from a moving-picture stock house at a reasonable price. The box is made up similar to the camera box, but with a metal back instead of the wood. The intense heat from the light would quickly burn the wood and for this reason the light should be kept from the film while it is not in motion. The projecting lens barrel should be fitted snugly, yet loose enough for focusing.
The baseboard is cut as shown and the film-stepping device is firmly attached to the small end. The sides extend over the baseboard and are fastened with screws and braced with metal brackets. The slot in the small end of the baseboard is for the film to pass through. The film should have a tension the same as in the camera with velvet placed on the edges of the partitions. It is well to have a guide below the roller shutter to keep the film from encircling the roller as it turns.