Who, I asked myself, was the scientific gunman?  I knew it was useless to try to hurry Garrick.  First, by a sort of intuition he had picked him out, then by the evidence of hammer and bullet he had made it practically certain.  But I knew that to his scientific mind nothing but absolute certainty would suffice.

While I was waiting for him to proceed, he had already begun to work on some apparatus behind a screen at the end of his office.  Close to the wall at the left was a stereopticon which, as nearly as I could make out, shot a beam of light through a tube to a galvanometer about three feet distant.  In front of this beam whirled a five-spindled wheel governed by a chronometer which was so accurate, he said, that it erred only a second a day.

Between the poles of the galvanometer was stretched a slender thread of fused quartz plated with silver.  It was the finest thread I could imagine, only a thousandth of a millimeter in diameter, far too tenuous to be seen.  Three feet further away was a camera with a moving plate holder which carried a sensitized photographic plate.  Its movement was regulated by a big fly-wheel at the extreme right.

“You see,” remarked Garrick, now engrossed on the apparatus and forgetting the hammer evidence for the time, “the beam of light focussed on that fine thread in the galvanometer passes to this photographic plate.  It is intercepted by the five spindles of the wheel, which turns once a second, thus marking the picture off in exact fifths of a second.  The vibrations of the thread are enormously magnified on the plate by a lens and produce a series of wavy or zigzag lines.  I have shielded the sensitized plate by a wooden hood which permits no light to strike it except the slender ray that is doing the work.  The plate moves across the field slowly, its speed regulated by the fly-wheel.  Don’t you think it is neat and delicate?  All these movements are produced by one of the finest little electric motors I ever saw.”

I could not get the idea of the revolvers out of my head so quickly.  I agreed with him, but all I could find to say was, “Do you think there was more than this one whom they call the Chief engaged in the shootings?”

“I can’t say absolutely anything more than I have told you, yet,” he answered in a tone that seemed to discourage further questioning along that line.

He continued to work on the delicate apparatus with its thread stretched between the stationary magnets of the galvanometer, a thread so delicate that it might have been spun by a microscopic spider, so light that no scales made by human hands could weigh it, so slender that the mind could hardly grasp it.  It was about one-third the diameter of a red corpuscle of blood and its weight had been estimated as about .00685 milligrams, truly a fairy thread.  It was finer than the most delicate cobweb and could be seen with the naked eye only when a strong light was thrown on it so as to catch the reflection.