"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.
"Well, the shadow in this picture can be made to prove an alibi for someone. Notice. It is seen prominently to the right, and its exact location on the house is an easy matter. The identification of the gable casting the shadow ought to be easy. To be exact, I have figured it out as 19.62 feet high. The shadow is 14.23 feet down, 13.10 feet east, and 3.43 feet north. You see, I am exact. I have to be. In one minute it moves 0.080 feet upward, 0.053 feet to the right, and 0.096 feet in its apparent path. It passes the width of a weatherboard, 0.37 foot, in four minutes and thirty-seven seconds."
Kennedy was talking rapidly of data which he had derived from the study of the photograph as from plumb line, level, compass, and tape, astronomical triangle, vertices, zenith, pole, and sun, declination, azimuth, solar time, parallactic angles, refraction, and a dozen other bewildering terms.
"In spherical trigonometry," he concluded, "to solve the problem three elements must be known. I know four. Therefore, I can take each of the known, treat it as unknown, and have four ways to check my result. I find that the time might have been either three o'clock, twenty-one minutes and twelve seconds in the afternoon, or 3:21:31 or 3:21:29, or 3:21:33. The average is 3: 21:26 and there can be no appreciable error except for a few seconds. I tell you that to show you how close I can come. The important thing, however, is that the date must have been one of two days, either May 22 or July 22. Between these two dates we must decide on evidence other than the shadow. It must have been in May, as the immature condition of the foliage shows. But even if it had been in July, that would be far from the date you allege. Why, I could even tell you the year. Then, too, I could look up the weather records and tell something from them. I can really answer, with an assurance and accuracy superior to the photographer himself, if you could produce him and he were honest, as to the real date. The original picture, aside from being doctored, was actually taken last May. Science is not fallible, but exact in this matter."
Kennedy felt that he had scored a palpable hit. Dorgan was speechless.
Still, Craig hurried on.