ll speed
northward, placing my private papers, the reports of Miss Barrison,
and your own reports in the hands of the authorities in Bronx Park.
Don't attempt to aid me; don't delay to search for me. Do you
promise?"

"Yes," we breathed together.

He looked at us solemnly. "If you fail me, you betray me," he said.

We swore obedience.

"Then let us begin," he said, and he rose and went to the steel cage.
Unlocking the door, he flung it wide and stepped inside, leaving the
cage door open.

"The moment a single pie is disturbed," he said to me, "I shall close
the steel door from the inside, and you and Miss Barrison will then
dump the rosium oxide and the strontium into the tank, clap on the
lid, turn the nozzle of the hose on the cage, and spray it
thoroughly. Whatever is invisible in the cage will become visible and
of a faint rose color. And when the trapped creature becomes visible,
hold yourselves ready to aid me as long as I am able to give you
orders. After that either all will go well or all will go otherwise,
and you must run for the launch." He seated himself in the cage near
the open door.

I placed the steel tank near the cage, uncoiled the hose attachment,
unscrewed the top, and dumped in the salts of strontium. Miss Barrison
unwrapped the bottle of rosium oxide and loosened the cork. We
examined this pearl-and-pink powder and shook it up so that it might
run out quickly. Then Miss Barrison sat down, and presently became
absorbed in a stenographic report of the proceedings up to date.

When Miss Barrison finished her report she handed me the bundle of
papers. I stowed them away in my wallet, and we sat down together
beside the tank.

Inside the cage Professor Farrago was seated, his spectacled eyes
fixed on the row of pies. For a while, although realizing perfectly
that our quarry was transparent and invisible, we unconsciously
strained our eyes in quest of something stirring in the forest.

"I should think," said I, in a low voice, "that the odor of the pies
might draw at least one out of the odd dozen that came rubbing up
against my window last night."

"Hush! Listen!" she breathed. But we heard nothing save the snoring of
the overfed dog at our feet.

"He'll give us ample notice by butting into Miss Barrison's skirts," I
observed. "No need of our watching, professor."

The professor nodded. Presently he removed his spectacles and lay back
against the bars, closing his eyes.

At first the