mark that pane, and
be ready to replace it when I give the word."

From the instrument case he had brought he took out a spectroscope. He
turned back the mattress and mounted it on the bedstead.

"Cover that pane," he directed.

Carnes did so, and the Doctor swung the receiving tube of the instrument
until it pointed at the covered pane. He glanced into the eyepiece, and
then held a tiny flashlight for an instant opposite the third tube.

"Uncover that pane," he said.

Carnes took down the glass plate and the Doctor gazed into the
instrument. He made some adjustments.

"Are you familiar with spectroscopy, Admiral?" he asked.

"Somewhat."

"Take a squint in here and tell me what you see."

* * * * *

The Admiral applied his eye to the instrument and looked long and
earnestly.

"There are some lines there, Doctor," he said, "but your instrument is
badly out of adjustment. They are in what should be the ultra-violet
sector, according to your scale."

"I forgot to tell you that this is a fluoroscopic spectroscope designed
for the detection of ultra-violet lines," replied Dr. Bird. "Those lines
you see are ultra-violet, made visible to the eye by activation of a
radioactive compound whose rays in turn impinge on a zinc blende sheet.
Do you recognize the lines?"

"No, I don't."

"Small wonder; I doubt whether there are a dozen people who would. I
have never seen them before, although I recognize them from descriptions
I have read. Bolton, come here. Sight along this instrument and through
that plate of glass which Carnes is holding and tell me what office that
window belongs to."

Bolton sighted as directed up at the side of the State, War and Navy
Building.

"I can't tell exactly at this time of night, Doctor," he said, "but I'll
go into the building and find out."

"Do so. Have you a flashlight?"

"Yes."

"Flash it momentarily out of each of the suspected windows in turn until
you get an answering flash from here. When you do, flash it out of each
pane of glass in the window until you get another flash from here. Then
come back and tell me what office it is. Mark the pane so that we can
locate it again in the morning."

* * * * *

"It is the office of the Assistant to the Adjutant General of the Army,"
reported Bolton ten minutes later.

"What is there in the room?"

"Nothing but the usual desks and chairs."

"I suspected as much