He was going over the photographs carefully.

“For instance,” he continued, “in order to solve the riddle of a crime, the detective’s first task is to study the scene topographically.  Plans and elevations of a room or house are made.  The position of each object is painstakingly noted.  In addition, the all-seeing eye of the camera is called into requisition.  The plundered room is photographed, as in this case.  I might have done it by placing a foot rule on a table and taking that in the picture.  But a more scientific and accurate method has been devised by Bertillon.  His camera lens is always used at a fixed height from the ground and forms its image on the plate at an exact focus.  The print made from the negative is mounted on a card in a space of definite size, along the edges of which a metric scale is printed.  In the way he has worked it out, the distance between any two points in the picture can be determined.  With a topographical plan and a metric photograph one can study a crime, as a general studies the map of a strange country.  There were several peculiar things that I observed at your house, Carton, and I have here an indelible record of the scene of the crime.  Preserved in this way, it cannot be questioned.  You are sure that the only thing missing is the photographs?”

Carton nodded, “I never keep anything valuable lying around.”

“Well,” resumed Kennedy, “the photographs were in this cabinet.  There are other cabinets, but none of them seems to have been disturbed.  Therefore the thief must have known just what he was after.  The marks made in breaking the lock were not those of a jimmy, but of a screwdriver.  No amazing command of the resources of science is needed so far.  All that is necessary is a little scientific common sense.”

Carton glanced at me, and I smiled, for it always did seem so easy, when Craig did it, and so impossible when we tried to go it alone.

“Now, how did the robber get in?” he continued, thoroughly engrossed in his study.  “All the windows were supposedly locked.  I saw that a pane had been partly cut from this window at the side—­ and the pieces were there to show it.  But consider the outside, a moment.  To reach that window even a tall man must have stood on a ladder or something.  There were no marks of a ladder or even of any person in the soft soil of the garden under the window.  What is more, that window was cut from the inside.  The marks of the diamond which cut it plainly show that.  Scientific common sense again.”

“Then it must have been someone in the house or at least familiar with it?” I exclaimed.

Kennedy shook his head affirmatively.

I had been wondering who it could be.  Certainly this was not the work of Dopey Jack, even if the far cleverer attempt on Langhorne’s safe had been.  But it might have been one of his gang.  I had not got as far as trying to reason out the why of the crime.