He paused.  Dorgan was regarding him keenly, but saying nothing.  Kennedy did not mind, as he resumed.

“Combination photographs change entirely the character of the initial negative and have been made for the past fifty years.  The earliest, simplest, and most harmless photographic deception is the printing of clouds in a bare sky.  But the retoucher with his pencil and etching tool to-day is very skilful.  A workman of ordinary ability can introduce a person taken in a studio into an open-air scene well blended and in complete harmony without a visible trace of falsity.”

Dorgan was growing interested.

“I need say nothing of how one head can be put on another body in a picture,” pursued Craig, “nor need I say what a double exposure will do.  There is almost no limit to the changes that may be wrought in form and feature.  It is possible to represent a person crossing Broadway or walking on Riverside Drive, places he may never have visited.  Thus a person charged with an offence may be able to prove an alibi by the aid of a skilfully prepared combination photograph.

“Where, then,” asked Kennedy, “can photography be considered as irrefutable evidence?  The realism may convince all, except the expert and the initiated after careful study.  A shrewd judge will be careful to insist that in every case the negative be submitted and examined for possible alterations by a clever manipulator.”

Kennedy bent his gaze on Dorgan.  “Now, I do not accuse you, sir, of anything.  But a photograph has come into my possession in which Mr. Carton is represented as standing in a group on a porch, with Mr. Murtha, Mrs. Ogleby, and an unknown woman.  The first three are in poses that show the utmost friendliness.  I do not hesitate to say that was originally a photograph of yourself, Mr. Murtha, Mrs. Ogleby, and a woman whom you know well.  It is a pretty raw deal, a fake in which Carton has been substituted by very excellent photographic forgery.”

“A fake—­huh!” repeated Dorgan, contemptuously.  “How about the story of them?  There’s no negative.  You’ve got to show me that the original print stolen from Carton, we’ll say, is a fake.  You can’t do it.  No, sir, those pictures were taken this summer.”

Kennedy quietly laid down the bundle of photographs copied from those alleged to have been stolen from Carton.  He was pointing to a shadow of a gable on the house.

“You see that shadow of the gable, Dorgan?” he asked.  “Perhaps you never heard of it, but it is possible to tell the exact time at which a photograph was taken from a study of the shadows.  It is possible in theory and practice, and it can be trusted absolutely.  Almost any scientist, Dorgan, may be called in to bear testimony in court nowadays, but you probably think the astronomer is one of the least likely.