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."

"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 coff