“Hello, here’s something!” cried Tom, as he got up from the operator’s seat, where he had taken his place to test the various controls.

“What is it?” asked Ned.

“A button.  A queer sort of a button.  I never had any like that on my clothes, and I’m sure you didn’t.  Look!” and Tom held out a large, metal button of curious design.

“It must have come off the coat of one of the men who had your airship, Tom,” said his chum.  “Save it.  You may find that it’s a clue.”

“I will.  No telling what it may lead to.  Well, I guess that’s all we can find.”

And it was.  But Tom little realized what a clue the button was going to be.  Nothing more could be learned by staring at the returned airship, so he and Ned went back to the house.

Tom Swift had many things to do, but his chief concern was for the photo telephone.  Now that he was near the goal of success he worked harder than ever.  The idea Ned had given him of being able to take the picture of a person at the instrument—­without the knowledge of that person—­appealed strongly to Tom.

“That’s going to be a valuable invention!” he declared, but little he knew how valuable it would prove to him and to others.

It was about a week later when Tom was ready to try the new apparatus.  Meanwhile he had prepared different plates, and had changed his wiring system.  In the days that had passed nothing new had been learned concerning the whereabouts of Mr. Damon, nor of the men who had so mysteriously taken away Tom’s airship.

All was in readiness for the trial.  Tom sent Ned to the booth that he had constructed in the airship hangar, some distance away from the house.  The other booth Tom had placed in his library, an entirely new system of wires being used.

“Now Ned,” explained Tom, “the idea is this!  You go into that booth, just as if it were a public one, and ring me up in the regular way.  Of course we haven’t a central here, but that doesn’t matter.  Now while I’m talking to you I want to see you.  You don’t know that, of course.”

“The point is to see if I can get your picture while you’re talking to me, and not let you know a thing about it.”

“Think you can do it, Tom?”

“I’m going to try.  We’ll soon know.  Go ahead.”

A little later Ned was calling up his chum, as casually as he could, under the circumstances.

“All right!” called Tom to his chum.  “Start in and talk.  Say anything you like—­it doesn’t matter.  I want to see if I can get your picture.  Is the light burning in your booth?”

“Yes, Tom.”

“All right then.  Go ahead.”

Ned talked of the weather—­of anything.  Meanwhile Tom was busy.  Concealed in the booth occupied by Ned was a sending plate.  It could not be seen unless one knew just where to look for it.  In Tom’s booth was a receiving plate.

The experiment did not take long.  Presently Tom called to Ned that he need stay there no longer.