“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.” “Why?” Again the doctor hesitated almost shamefacedly. “It’s so--so queer,” said he. “I can’t expect you to believe it. I didn’t believe it myself till the chief made me examine the marks under the magnifier and again under his pocket microscope. It was a tennis shoe. Of course Quimby began to ransack Thaxton Vail’s boot trees and to compare his soles with the size of this. Well, the sole-mark on the sill was fully two sizes larger than any of Thaxton’s soles.” “I don’t see anything unbelievable about that,” she commented. “It clears Thax all the more completely.” “You’re right,” said Lawton. “It clears Thax all right as far as it goes. But that isn’t the unbelievable part of it. There was a pair of tennis shoes under the edge of the bed. Lying a yard or so apart and in the shadow. We none of us saw them first on account of the light. Not till we had tested all Vail’s shoes by that imprint on the sill. Then the chief hit his toe against one of them. He stooped down and hauled them out. They had bits of mud still sticking to their instep. But the left one had much less than the other. They were bigger than any of Vail’s shoes. But we didn’t notice that till we had tested the left one--the one with the least mud on it--against the sill’s imprint. It fitted exactly. It did more. The sole-grips were new rubber with a funny crisscross pattern. And those grips were precisely the same as the marks on the sill. The microscope proved it. The step on the sill was made by that very shoe. There couldn’t be any doubt of it.” “But--” “Then came the oddest part,” continued the doctor. “You’ve seen Cooley, the night constable? He clerks, part-time, in the new shoe store they’ve opened this year at Aura. And he grabbed hold of those tennis shoes and gave them one good look. Then he vowed they are a pair his boss had sent for--all the way from New York--to a pedic specialist--for Willis Chase.” “_What?_”