s
pillow. I'll show you how to work it. You know that when light is
reflected the angle of reflection always equals the angle of incidence?
Well, you place these three feet against the pane of glass, thus putting
the base of the instrument in a plane parallel to the pane of glass. By
turning these two knobs, one of which gives lateral and the other
vertical adjustment, you will manipulate the instrument until the first
telescope is pointing directly toward the President's pillow. Now notice
that the two telescope barrels are fastened together and are connected
to the knobs, so that when the knobs are turned, the scopes are turned
in equal and opposite amounts. When one is turned from its present
position five degrees to the west, the other automatically turns five
degrees to the east. When one is elevated, the other is correspondingly
depressed. Thus, when the first tube points toward the pillow, the other
will point toward the source of the reflected beam."

"Clever!" ejaculated Bolton.

"It is rather crude and may not be accurate enough to locate the source
exactly, but at least it will give us a pretty good idea of where to
look. Given time, a much more accurate instrument could have been made,
but two telescopic rifle sights and a theodolite base were all the
materials I could find to work with. Climb out, Carnesy, and do your
stuff."

* * * * *

Carnes climbed out on the window and fastened the hooks of the life
saver to the rings set in the window casings. He sat the base of the
instrument against the pane of glass and manipulated the telescope knobs
as Dr. Bird signalled from the inside. The scientist was hard to please
with the adjustment, but at last the cross hairs of the first telescope
were centered on the light in the solarium. He changed his position and
stared through the second tube.

"The angle is too acute and the distance too great for accuracy," he
said with an air of disappointment. "The beam comes from the roof of a
house down along Pennsylvania Avenue, but I can't tell from here which
one it is. Take a look, Bolton."

The Chief of the Secret Service stared through the telescope.

"I couldn't be sure, Doctor," he replied. "I can see something on the
roof of one of the houses, but I can't tell what it is and I couldn't
tell the house when I got in front of it."

"It won't do to make a false move," said the Doctor. "Did you arrange
for that plane?"

"It is w