As before, there was an extension telephone, so that Mrs. Damon could talk to the unknown, while Tom could hear as well.  But he planned to take no part in the conversation unless something unforeseen occurred.

Mr. Damon was an enthusiastic photographer, and he had a dark room adjoining his library.  It was in this dark room that Tom planned to develop the photo telephone plate.

On this occasion he was not going to use the metal plate in which, ordinarily, the image of the person talking appeared.  That record was but a fleeting one, as in a mirror.  This time Tom wanted a permanent picture that could, if necessary, be used in a court of justice.

Tom’s plan was this:  If the person who had demanded the papers came to one of the photo telephones, and spoke to Mrs. Damon, Tom would switch on the receiving apparatus.  Thus, while the man was talking, his picture would be taken, though he would not know of the thing being done.

His voice would also be recorded on the wax cylinder, and he would be equally unaware of this.

When Tom had imprinted the fellow’s image on the prepared plate, he would go quickly to the dark room and develop it.  A wet print could be made, and with this as evidence, and to use in identification, a quick trip could be made to the place whence the man had telephoned.  Tom hoped thus to capture him.

To this end he had his airship in waiting, and as soon as he had developed the picture he planned to rush off to the vicinity of the sawmill, and make a prisoner of the man whose features would be revealed to him over the wire.

It was a hazardous plan—­a risky one—­but it was the best that he could evolve.  Tom had instructed Mrs. Damon to keep the man in conversation as long as possible, in order to give the young inventor himself time to rush off in his airship.  But of course the man might get suspicious and leave.  That was another chance that had to be taken.

“If I had thought of it in time,” said Tom, musingly, as he paced up and down in the library waiting for the ’phone to ring, “if I had thought of it in time I would have rigged up two plates—­one for a temporary, or looking-glass, picture, and the other for a permanent one.  In that way I could rush off as soon as I got a glimpse of the fellow.  But it’s too late to do that now.  I’ll have to develop this plate.”

Waiting is the most wearisome work there is.  Tom and Ned found this to be the case, as they sat there, hoping each moment that the telephone bell would ring, and that the man at the other end of the wire would be the mysterious stranger.  Mrs. Damon, too, felt the nervous strain.

“This is about the hour he called up yesterday,” said Tom, in a low voice, after coming back from a trip to the window to see that his airship was in readiness.  He had brought over to help in starting it, for he was using his most powerful and speedy craft, and the propellers were hard to turn.