"The speed. You know how fast an ordinary movie is taken, don't you? No?

Well, it's sixteen exposures per second. The slow pictures are taken sometimes at a hundred and twenty-eight or two hundred and fifty-six exposures per second, and then shown at sixteen. This affair will take half a million pictures per second."

"I didn't know that a film would register with that short an exposure."

"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 pa.s.s 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 gla.s.ses 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."

"It's too deep for me," said Carnes with a sigh. "Can I do anything to help? The doctor told me to stand by and do anything I could."

"I might be able to use you a little if you can use tools," said Casey with a grin. "You can start bolting together that light proof shield if you want to."

"Well, Carnes, did you have an instructive night?" asked Dr. Bird cheerfully as he entered the First National Bank at eight-thirty the next morning.

"I don't see that I did much good, Doctor. Casey would have had the machine ready on time anyway, and I'm no machinist."

"Well, frankly, Carnes, I didn't expect you to be of much help to him, but I did want you to see what Casey was doing, and a little of it was pretty heavy for him to handle alone. I suppose that everything is ready?"

"The motor reached full speed about fifteen minutes ago and Casey went out to get a cup of coffee. Would you mind telling me the object of the whole thing?"

"Not at all. I plan to make a permanent record of the work of the most ingenious bank robber in the world. I hope he keeps his word."

"What do you mean?"

"Three days ago when Sturtevant sweated a 'confession' out of poor Winston, the bank got a message that the robbery would be repeated this morning and dared them to prevent it. Rogers thought it was a hoax, but he telephoned me and I worked the Bureau men night and day to get my camera ready in time for him. I am afraid that I can't do much to prevent the robbery, but I may be able to take a picture of it and thus prevent other cases of a like nature."

"Was the warning written?"

"No. It was telephoned from a pay station in the loop district, and by the time it was traced and men got there, the telephoner was probably a mile away. He said that he would rob the same cage in the same manner as he did before."

"Aren't you taking any special precautions?"

"Oh, yes, the bank is putting on extra guards and making a lot of fuss of that sort, probably to the great amus.e.m.e.nt of the robber."

"Why not close the cage for the day?"

"Then he would rob a different one and we would have no way of photographing his actions. To be sure, we will put dummy money there, bundles with bills on the outside and paper on the inside, so if I don't get a picture of him, he won't get much. Every bill in the cage will be marked as well."

"Did he say at what time he would operate?"

"No, he didn't, so we'll have to stand by all day. Oh, h.e.l.lo, Casey, is everything all right?"

"As sweet as chocolate candy, Doctor. I have tested it out thoroughly, and unless we have to run it so long that the film wears out and breaks, we are sitting pretty. If we don't get the pictures you are looking for, I'm a dodo, and I haven't been called that yet."

"Good work, Casey. Keep the bearings oiled and pray that the film doesn't break."

The bank had been opened only ten minutes when the clangor of gongs announced a robbery. It was practically a duplicate of the first. The paying teller had turned from his window to take some bills from his rack and had found several dozens of bundles missing. As the gongs sounded, Dr. Bird and Casey leaped to the camera.

"She snapped, Doctor!" cried Casey as he threw two switches. "It'll take an hour to stop and half a day to develop the film, but I ought to be able to show you what we got by to-night."

"Good enough!" cried Dr. Bird. "Go ahead while I try to calm down the bank officials. Will you have everything ready by eight o'clock?"

"Easy, Doctor," replied Casey as he turned to the magnetic brake.

By eight o'clock quite a crowd had a.s.sembled in a private room at the Blackstone Hotel. Besides Dr. Bird and Carnes, Rogers and several other officials of the First National Bank were present, together with Detective-Captain Sturtevant and a group of the most prominent scientists and physicians gathered from the schools of the city.

"Gentlemen," said Dr. Bird when all had taken seats facing a miniature moving picture screen on one wall, "to-night I expect to show you some pictures which will, I am sure, astonish you. It marks the advent of a new departure in transcendental medicine. I will be glad to answer any questions you may wish to ask and to explain the pictures after they are shown, but before we start a discussion, I will ask that you examine what I have to show you. Lights out, please!"

He stepped to the rear of the room as the lights went out. As his eyes grew used to the dimness of the room he moved forward and took a vacant seat. His hand fumbled in his pocket for a second.

"Now!" he cried suddenly.

In the momentary silence which followed his cry, two dull metallic clicks could be heard, and a quick cry that was suddenly strangled as Dr. Bird clamped his hand over the mouth of the man who sat between him and Carnes.

"All right, Casey," called the doctor.

The whir of a projection machine could be heard and on the screen before them leaped a picture of the paying teller's cage of the First National Bank. Winston's successor was standing motionless at the wicket, his lips parted in a smile, but the attention of all was riveted on a figure who moved at the back of the cage. As the picture started, the figure was bent over an opened suitcase, stuffing into it bundles of bills. He straightened up and reached to the rack for more bills, and as he did so he faced the camera full for a moment. He picked up other bundles of bills, filled the suitcase, fastened it in a leisurely manner, opened the rear door of the cage and walked out.

"Again, please!" called Dr. Bird. "And stop when he faces us full."

The picture was repeated and stopped at the point indicated.

"Lights, please!" cried the doctor.

The lights flashed on and Dr. Bird rose to his feet, pulling up after him the wilted figure of a middle-aged man.