The doctor waited until the trio passed out of earshot. Then, lowering his voice, he said quizzically:
“The chief’s got another bee in his bonnet now. He’s all up in the air over it. He says it lands the case against a blank wall.”
“What do you mean?” she asked, puzzled at his hint.
“Why,” said the doctor, as if ashamed to mention so fantastic a thing, “you know there was a shoe mark on the window-sill and a scrap of mud where the killer had stepped on the sill on the way out.”
“Or in,” suggested Doris.
“Out,” corrected Lawton.
“How do you know?”
“The chief put his magnifying glass over it in the strong light just now,” said Dr. Lawton. “Then he made us all take a peep. There was a faint outline of the ball of a shoe pressed against the white woodwork of the sill. And the shoe faced outward. That was clear from the curve of its outer edge. It was a left foot at that. A tennis shoe.”
“He wore tennis shoes to muffle the sound of his steps?” cried Doris.
“That’s what I thought first,” answered Lawton. “So did the chief. But we both changed our minds.”