Page:Popular Science Monthly Volume 88.djvu/468

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HE motion picture camera shown in the drawing is very simple in construction and operation. It holds standard film rolls and is about 5″ by 7″ by 8″ in dimensions.

The film passes from the upper magazine over the toothed spool, down through the slot where the exposure is made (size of exposure ¾″ high by 1″ wide) and then over the lower toothed spool on to the take-up reel, which is keyed to the shaft on which it rests. The shaft in turn is connected through gears to a clock-spring. This gives the reel the power to take up the exposed film as used.

It will be noticed that the lower toothed spool has a four-toothed gear fastened to its shaft. The action of the large wheel, which contains the four pegs, on the four-toothed gear is similar to the Geneva movement on most motion picture projectors. This large wheel is driven from the crank by four to one gearing, and as each of the pegs turns over four teeth of the little spool, the height of one exposure or ¾″, sixteen exposures are made to one revolution of the crank. Two little springs rub on the toothed spool to prevent slipping of the film in either direction, which action should take approximately one second.

A universal-focus lens is shown in the drawing, but a focusing lens may be used, in which case the shutter must be placed behind. The shutter is of the semicircular revolving type, driven through the chain and gears from the crank at a ratio of sixteen to one, or sixteen revolutions of the shutter to one of the crank. This will make a revolution of the shutter to each exposure. By shifting the chain forward or backward, the shutter can be made to uncover at the proper moment; that is, just after the fresh section of film has come to rest.

Before using the camera the spring must be wound. A cover should be kept over the lens.—

HE ordinary envelope when sealed can very easily be opened and resealed, and the chances of detection are rather slight, especially if care be taken when resealing to see that the flap is put back in the exact position it first occupied. The attached drawings illustrate a distinct improvement on the old style flap. Instead of rounding off into a point, it is extended into a narrow strip, the length of this strip being the exact difference between the rounded point of the old-fashioned flap, when sealed, and the bottom of the envelope. A slit is cut in the back of the envelope, a little wider (1/16″) than the width of this strip, half way between where the rounded point would come and the bottom of the envelope. The flap is gummed in the ordinary way, and the extra strip is gummed on the lower half of the opposite side. Figure 1.

The envelope is sealed as usual. The gum on the lower outside half of the strip is dampened, and the strip is easily slid into the slit in the envelope and pressed down, sealing it to the inside of the envelope, Figure 2. Opening and resealing this envelope, undetected, is practically impossible.—