"That's slow," replied Casey with a laugh. "It all depends on the light. The best flash-light powder gives a flash about one ten-thousandth of a second in duration, but that is by no means the speed limit of the film. The only trouble is enough light and sufficient shutter speed. Pictures have been taken by means of spark photography with an exposure of less than one three-millionth of a second. The whole secret of this machine lies in the shutter. This big disc with the slots in the edge is set up before the lens and run at such a speed that half a million slots per second pass before the lens. The film, which is sixteen millimeter X-ray film, travels behind the lens at a speed of nearly five miles per second. It has to be gradually worked up to this speed, and after the whole thing is set up, it takes it nearly four hours to get to full speed." "At that speed, it must take a million miles of film before you get up steam." "It would, if the film were being exposed. There is only about a hundred yards of film all told, which will run over these huge drums in an endless belt. There is a regular camera shutter working on an electric principle which remains closed. When the switch is tripped, the shutter opens in about two thirty-thousandths of a second, stays open just one one-hundredth of a second, and then closes. This time is enough to expose nearly all of our film. When we have our picture, I shut the current down, start applying a magnetic brake, and let it slow down. It takes over an hour to stop it without breaking the film. It sounds complicated, but it works all right." "Where is your switch?" * * * * * "That is the trick part of it. It is a remote control affair. The shutter opens and starts the machine taking pictures when the back door of the paying teller's cage is opened half an inch. There is also a hand switch in the line that can be opened so that you can open the door without setting off the camera, if you wish. When the hand switch is closed and the door opened, this is what happens. The shutter on the camera opens, the machine takes five thousand pictures during the next hundredth of a second, and then the shutter closes. Those five thousand exposures will take about five minutes to show at the usual rate of sixteen per second." "You said that you had to get plenty of light. How are you managing that?" "The camera is equipped with a special lens ground out of rock crystal. This lens lets in ultra-violet light which the ordinary lens shuts out, and X-ray film is especially sensitive to ultra-violet light. In order to be sure that we get enough illumination, I will set up these two ultra-violet floodlights to illumine the cage. The teller will have to wear glasses to protect his eyes and he'll get well sunburned, but something has to be sacrificed to science, as Dr. Bird is always telling me."