Mendel, still in his woolen slippers, cried out, "Has anyone been along here in the last hour?"

"No Herr Oberst," said the guard, coming rapidly to attention as he saw his superior officer. "No one, it has been quiet since before lunch."

Everyone pushed past him and almost ran to the broken door leading into the infamous guest room. There was a crush in the doorway, but as they all gained entrance what they saw was a fire on the desk by the window. They stopped and looked in amazement.

Burning on a metal baking tray taken from Brother Victor's kitchen were a pile of papers. Black smoke drafting away from the fire and out of the window made it hard to see at first, so Mendel, who knew what was happening, stepped from behind the group, went over to the fire, now almost out, and patted the embers into submission.

"I think you will agree that Brother Matthew has been with us for the whole afternoon?" he said, almost pleasantly, but his friends caught the inflection in his voice and prayed that the monk's almost pathological nervousness at times of high stress would not ruin the next few minutes.

"As you can see," he went on, "a fire has just started in this room, yet the guard outside swears that no one came in here during the last hour. How can this be?"

"Brother Gregory, what is this? What are you doing?" Abbot Napp was the first to recover from his shock, but behind him Monsignor Schrattenbach was already smiling.

"Just giving you a demonstration," Mendel said, reaching down and picking up more crumpled papers from the floor beside the desk. "Please watch." He placed the papers on the baking tray, sprinkled a small amount of powder into a tiny pile on one of the crumpled papers, then took the antique reading glass and stand and brought it closer to the tray. As the astounded judges watched, Mendel adjusted the glass lens so that the afternoon rays of the sun shone directly through it.

As he had known would happen, the convex lens focused the sunlight into a concentrated spot right at the focal length of the lens. This concentrated spot was clearly visible and he moved the lens until it shone on the tiny pile of powder, which promptly began to smolder. Within seconds there was smoke coming from it, and shortly thereafter the crumpled paper also caught alight and flames once again flared up on the desk top.

how the fire started

Brother Gregory doused them with a handy wet cloth, and turned to his shocked audience. "You see," he said to them, "this is how the fire was started that day. Someone placed several pages from the conscription lists on the desk and then adjusted this reading glass lens so that the afternoon sun would shine through it, and start the fire. In just the way you have seen."

He continued, "Anyone could have set up the papers and the lens at any time that day - as soon as they knew it was going to be sunny - and then arranged an alibi. I seem to remember that the army was putting on a demonstration that day, so there would be lots of witnesses. While he was making sure he was seen at a great distance from this room, the sun was doing his work for him."

Mendel picked up the antique reading glass. "This lens has the property of concentrating the sun's rays until they form a very hot spot. By arraigning the lens and papers in just the right way, he could be sure that the sun would not start the fire until the afternoon, when he knew it shone in these windows. That is why he moved the desk to this position."

Darmstaedter was making croaking noises by now, and Brother Matthew slapped him on the back, allowing Mendel to continue.

"So, this is how the crime was committed, and no longer do we have to think in terms of who was here when it started. Anyone could have arranged the lens in this way, anyone could have started the fire, and anyone could have arranged destroy the lists. Brother Matthew was here at the monastery, so why would he go to all these elaborate lengths? No, the real guilty person was someone who had arranged to be far away when the fire started. We must re-open the investigation and see who else had a motive for starting the fire."