440 



Popular Science Monthly 



Home-Made Motion Picture Camera 



THE motion picture camera shown in 

 the drawing is very simple in con- 

 struction and operation. It holds stand- 

 ard film rolls and is about 5'' by 7'' by 

 8" in dimensions. 



The film passes from the upper mag- 

 azine over the toothed spool, down 



The working mechanism of a home- 

 made motion picture camera 



through the slot where the exposure is 

 made (size of exposure %'' high by V 

 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, w^hich contains the four 

 pegs, on the four-toothed gear is similar 

 to the Geneva movement on most mo- 

 tion 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 %", six- 

 teen exposures are made to one revolu- 

 tion of the crank. Two little springs rul) 

 on the toothed spool to prevent slipping 

 of the film in either direction, which ac- 

 tion 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 six- 

 teen 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. — E. G. Gettins. 



The Flap-Lock Envelope 



THE ordinary envelope when sealed 

 can very easily be opened and re- 

 sealed, 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 occu- 

 pied. 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 dif- 

 ference 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 



JiUMMECl SACK 



This envelope can- 

 not be secretly 

 opened without 

 certain detection 



FIG I riG-I 



ordinary way, and the extra strip is 

 gummed on the lower half of the oppo- 

 site 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 re- 

 sealing this envelope, undetected, is 

 practically impossible.— J. A. McManus. 



