. The window is merely a reflector. That is all that
we can do for to-night, gentlemen. Admiral, keep your patient quiet and
in a room with _glass_ windows, preferably with the shades drawn, until
further notice. Bolton, meet me here with Carnes at sunrise. Have a
picked detail of ten men standing by where we can get hold of them in a
hurry. In the mean time, get the Chief of Air Service out of bed and
have him order a plane at Langley Field to be ready to take off at 6
A. M. He is not to take off, however, until I give him orders to do so.
Do you understand?"

"Everything will be ready for you, Doctor, but I confess that I don't
know what it is all about."

"It's the biggest case you ever tackled, old man, and I hope that we can
pull it off successfully. I'd like to go over it with you now, but I'll
be busy at the Bureau for the rest of the night. Drop me off there, will
you?"

At sunrise the next morning, Bolton met Dr. Bird at the entrance to the
White House grounds.

"Where is your detail?" he asked.

"In the State, War and Navy Building."

"Good. I want to go to the solarium, put a light on the place where the
President's pillow was last night, and mark that pane of quartz we were
looking through. Then we'll join the detail."

* * * * *

Dr. Bird placed the light and walked with Carnes across the White House
grounds. Bolton's badge secured admission to the State, War and Navy
Building for the party and they made their way to the office of the
Assistant to the Adjutant General.

"Did you mark the pane of glass through which you flashed your light
last night, Bolton?" asked the Doctor.

The detective touched one of the panes.

"Good," exclaimed the Doctor. "I notice that this window has hooks for a
window washer's belt. Get a life belt, will you?"

When the belt was brought, the Doctor turned to Carnes.

"Carnes," he said, "hook on this life saver and climb out on the window
ledge. Take this piece of apparatus with you."

He handed Carnes a piece of apparatus which looked like two telescopes
fastened to a base, with a screw adjustment for altering the angles of
the barrels.

* * * * *

Carnes took it and looked at it inquiringly.

"That is what I was making at the Bureau last night," explained Dr.
Bird. "It is a device which will enable me to locate the source of the
beam which was reflected from this pane of glass onto the President'