on the
spy's face.

The next instant the crackle of military commands and the thud of
footsteps burst upon them. The four--Professor Paulsen, Major Coggleston
and the spy, in a heap on the ground; and Henry Horn, wide-eyed and
trembling, standing near at hand--were illumined in a powerful
flashlight's beam. Half a dozen soldiers rushed up.

"Major! We heard that shot! Are you all right?"

The officer struggled to his feet, trying hard to preserve the dignity
of his rank despite his nudity. In the light of the flash he looked even
more than before like an overgrown kewpie doll.

"Of course I'm all right!" he puffed. "What's more, that red-headed rat
on the ground is the spy and murderer we've been looking for. Take him
away, men!"

He turned to Professor Paulsen.

"Joe, this is one time I don't know what to say. If it hadn't been for
you that devil would have made a clean getaway."

"Forget it," retorted the gaunt scientist. "It's little enough I can do
for my country at my age."

"Honestly, Joseph, I can't see how you got the nerve to do it!" marveled
Henry, still wide-eyed. "Just think, we might all have been killed--"

The professor glared.

"What do you mean, we might all have been killed?"

"Why, the explosive in that package, and the detonator--really, Joseph,
it was terribly dangerous--"

"Dangerous!" snorted the savant. "The only dangerous part was that he
might have hit me over the head with it."

"But--the explosive--"

"Explosive, my eye!" And, again glaring: "Do you mean to tell me you
can't understand why that stuff he had in the package didn't go off, you
abbreviated atom?"

Henry's goatee waggled uncertainly. He adjusted the steel-rimmed
spectacles which were his only garment.

"Well ... really, Joseph...."

"I'll admit right out I don't get it," broke in Major Coggleston. "You
mean there wasn't any danger of that stuff going off?"

"Of course not." Professor Paulsen was distinctly snappish.

"But why--"

* * * * *

The scientist turned back to Henry. "Don't you remember what I said to
you this morning about those devil's glasses of yours transposing
letters instead of just reversing them? And that you told me it would
take a special lens to straighten them out?"

"You mean--"

"Take any formula and transpose the symbols all the way through, and see
what you get. Trinitrocresol, for instance. The formula is
C_{7}H_{5}N_{3}O_{7}. Transpose it a