"The stuff is face powder," Black Hood said. "But it's something else, too. Mixed in with the face powder is another substance."
"Naphthionate of sodium," the Hermit said.
"That's what I thought," Black Hood nodded. "It's one of those substances which becomes phosphorescent in ultra-violet light. And those light bulbs in Jack Carlson's garage were ultra-violet bulbs. The light from them is invisible to us poor mortals. You see what that means, Hermit?"
"Not entirely," the Hermit said.
"It means that Jack Carlson was marked for murder. That face powder came from the cheek of a woman—some woman who pressed her cheek against Carlson's lapel. And a pretty gesture of affection it was, too. It made Carlson so easy to kill!
"You see, the naphthionate of sodium in that powder sticks to just about anything. Even if Carlson had brushed the face powder off, the naphthionate would still have been there. When Carlson entered the garage, he turned on the light switch. No visible light came from those bulbs—only "black light" as it is called. And the killer was waiting. In the black light, the killer could not be seen, but Carlson was perfectly targeted by that smudge of naphthionate which glowed on his lapel.
"It was all planned in advance—the lady's part to smear the powder on Carlsons' lapel, a sort of Judas kiss. And then there was the killer's part—to replace the ordinary bulbs with the ultra-violet type, and to wait with drawn gun to shoot Carlson."
"Who, then, is the Eye?" the Hermit asked.
"I'll stick to my original idea," Black Hood said after a moment's thought. "I still think that Jack Carlson is—was—the Eye. That alibi he arranged for himself at Weedham's home, that warning from the Eye which stated that Carlson was to die, his efforts to make Biggert's death look as though the killer had been shooting at Carlson instead of at Biggert—that all points to Carlson as the Eye. He was trying to make himself appear the fair-haired boy in front of Sergeant McGinty.
"Further, and I think conclusive proof, is that signal device which was used to 'warn' Carlson. That was—Carlson's own device. It was Vida Gervais, I believe, who turned the signal light through the French windows at the Weedham house. And then later, in a previously appointed spot, she left the signal light for Carlson to pick up as he left the house.