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VII THE MOVING PICTURE CAMERA 71 five. It is essential that the handle should be rotated at uniform speed, and occasionally it is no easy matter to resist the desire to slow down. For instance, when a funeral is passing, there is an almost irresistible inclina- tion to slacken speed. The amateur responds to this desire, and is astonished at the dislocated character of his pictures when projected upon the screen. As a matter of fact, the slowest funeral procession has to be taken with the same number of revolutions per second, and the same number of exposures-^sixteen pictures per second—as an express train travelling at 60 miles an hour; the crawl of the tortoise has to be photographed just as rapidly as a horse race, in order to secure natural motion. If a subject has been photogr,aphed too slowly it cannot be corrected by accelerating the speed in projection. The judgment of the actinic value of the sunlight, and the requisite stop for the lens; as well as the adjustment of the shutter, are factors which only can be determined from experience. Possibly the box, A contains a roll of film measuring 300 feet, whereas the subject photographed only demands the exposure of 60 or 100 feet. Then one makes a few punch marks on the film by the manipulation of a sma'l brass knob projecting from the outside of the camera, and marked "film punch." When the film reaches the de- veloping room, the operators working in the ruby light can tell by these marks the ends of a series of exposures. When the upper box has been exhausted of film, the lower box, now charged with exposed film, is removed, the einpty upper box is slipped out of its compartment and re-inserted into the lower recess, to serve in turn as a receptacle for film after exposure, and a fresh loaded film-box is placed in the upper space, the mechanism being threaded as before. The operator can ascertain at a glance how much film still remains to be used, because on the outside of the case is a dial which registers in feet the amount of film exposed, and which is set to zero whenever a fresh box of film is inserted. The task of the operator is increased in difficulty where the subject being taken is moving at great speed hori-