“Oh, come, we must play the game accordin’ to rules,” said Lord John. “It’s old man Challenger’s show and we are here by his good will, so it would be rotten bad form if we didn’t follow his instructions to the letter.”

“A pretty business it is!” cried the Professor, bitterly. “It struck me as preposterous in London, but I’m bound to say that it seems even more so upon closer acquaintance. I don’t know what is inside this envelope, but, unless it is something pretty definite, I shall be much tempted to take the next downriver boat and catch the Bolivia at Pará. After all, I have some more responsible work in the world than to run about disproving the assertions of a lunatic. Now, Roxton, surely it is time.”

“Time it is,” said Lord John. “You can blow the whistle.” He took up the envelope and cut it with his penknife. From it he drew a folded sheet of paper. This he carefully opened out and flattened on the table. It was a blank sheet. He turned it over. Again it was blank. We looked at each other in a bewildered silence, which was broken by a discordant burst of derisive laughter from Professor Summerlee.