ey. "He thinks he will need your machine to-morrow."

"I'll have it ready to turn on the power at four A.M.," replied Casey.

Carnes watched him curiously for a while as he soldered together the
electrical connections and assembled an apparatus which looked like a
motion picture projector.

"What are you setting up?" he asked at length.

"It is a high speed motion picture camera," replied Casey, "with a
telescopic lens. It is a piece of apparatus which Dr. Bird designed
while he was in Washington last week and which I made from his sketches,
using some apparatus we had on hand. It's a dandy, all right."

"What is special about it?"

"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 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