even been opened and examined by the police, who had carefully noted
everything else in the library. But as soon as the box was missed
strange questions began to be asked and conjecturally answered. The
police for the first time then observed that though it was half-past
eight at night when the murder occurred, and the lamp was not
lighted, the witnesses who burst first into the room described all
they saw as if they had seen it clearly. They spoke of things as
they would be seen in a very bright light, with absolute
definiteness. This set up inquiry, and the result of the inquiry was
to bring out the fact, which in the excitement of the moment had
escaped the notice of all the servants, that as they entered the
room and stared about at the murder, the electric flash of the
apparatus was actually in operation. But the scene itself had
diverted their attention from the minor matter of the light that
showed it.

The Inspector had been watching me narrowly as I read these
extracts. When I reached that point, he broke in with a word of
explanation.

"Well, that put me on the track, you see," he said, leaning forward
once more. "I thought to myself, if the light was acting, then the
whole apparatus must necessarily have been at work, and the scene as
it took place must have been photographed, act by act and step by
step, exactly as it happened. At the time the murderer, whoever he
was, can't have known the meaning of the flashes. But later, he must
have come to learn in some way what the electric light meant, and
must have realised, sooner than we did, that therein the box, in the
form of six successive negatives of the stages in the crime, was the
evidence that would infallibly convict him of this murder." He