Johnny did look, and there, to his vast astonishment, he beheld quite clearly outlined the footprint of a man.
“The sole of a heavy shoe!” He was dumbfounded. “It was not there before! And see! There are the marks of the nails; a heavy workman’s shoe. You could count those nails. And a heel of some hard, prepared stuff with the maker’s name wrong way over.”
“Yes,” Howe added quietly, “and a deep, jagged cut across the sole. Slipped on some sharp stone perhaps, when the sole was wet. That marks the shoe. It’s not like any other shoe in the world. Find that shoe, find the man who wears it, and we have made a discovery of great importance.”
“But I don’t understand!” Johnny was puzzled. “That mark was not there a moment ago.”
“Nor is it now.” Tom Howe chuckled. The violet light faded; the brilliant white light flashed on. The footprint was gone.
“Magic!” Johnny murmured. “Some form of magic. You can’t convict a man with magic.”
“Not magic, but science!” Howe’s tone was impressive. “Crooks are learning to use science as an aid in committing crimes. We must use science to detect crimes.”
Once again the white light was gone; the violet light returned and with it that mysterious vanishing footprint.
“You see,” Howe explained, “when that fellow entered the car to assist in carrying the famous football star away, he had been walking over a surface that contained some chemical solution. If he had passed over damp coffee grounds, or through a forest where rank vegetable matter rotted, the effect would have been as you see it now. His foot would have left a mark invisible in white light, but quite clearly outlined when subjected to the rays of an ultra-violet lamp.
“This lamp,” he went on, “has detected the secret writing of many a spy and jailbird. A spy, wishing to forward a secret report, dips his pen into a liquid made by soaking a few quinine pills in water. This writing will not show in white light until it has been treated. He writes some commonplace letter over this message and sends it forth. Our Secret service men seize it, put it under the ultra-violet lamp, and there it is; you can read every word. The moment the lamp is snapped off the message is gone.