echanism of unbelief. Good thing, or he
might have been the first to disappear.

"What's the matter with you, Martin? Can't you even answer when--" The
rest of what his wife was saying faded in the startled realization that
he was eating dinner.

"Sorry, dear," he murmured. "I was thinking." He was trying to recall
something that might tell him what day it was. It was obviously evening
or they wouldn't be eating dinner. "Uh," he said casually, "what day is
today?"

"Saturday," Fred said.

"Now Fred, don't tease your father about his absent-mindedness. This is
Thursday."

Thursday! That was right. He had given the lecture on the necessity of
unbelief today. There was tomorrow, when he could see if any of the
class had disappeared yet. He couldn't be certain, of course. Just
because a student didn't show up didn't mean he or she had vanished.

He fixed his eyes on Fred, across the table, and smiled. Fred, at least,
was a source of comfort. He knew the theory and hadn't vanished.

"Dad," Fred said. "I've been wondering if you saw a point of similarity
in the two disappearances?"

Martin thought, good heavens, does he have any inkling of what I've been
thinking? Of course not! He's just fumbling. Better to discourage him.
"Sorry, son. There aren't any similarities except accidental ones. I've
had the confidence of the police on this. The cases are quite
unrelated."

Fred refused to be sidetracked. "Dr. Henderson's face lit up as though a
sudden idea had struck him. I talked with some of his students. That's
what they all thought. And Horace Smith shouted to his wife, 'Ethel!
I've got it!' The next instant in each case they vanished into thin
air."

"But that doesn't mean a thing."

* * * * *

In the privacy of his study Martin Grant allowed himself to become
excited. Fred had unwittingly come upon the vital clue to the two
disappearances.

"Let's be clear about this," he said to himself, drumming on his desk
nervously with his fingers. "Undoubtedly there's a connection between
the vanishing and my theory. Both Horace and John arrived at something
I've missed. And since my theory is exhaustive it can't be there. It
must be--yes--it _must_ be that they went a step farther." He pondered
this a moment and added grudgingly, "A step I have missed." Then even
more grudgingly, "An obvious step."

Automatically he opened a drawer and brought out a sheet of paper and a
pencil. He wro