"An excellent idea, Vernet; but how, may I ask, will you know the car, when it passes you? There are hundreds of cars on the Avenue du Bois de Boulogne, at eight o'clock in the evening."
Vernet laughed. "I confess, Monsieur, you have me there."
"Of course you might station a man at the intersection of the Avenue Malakoff and the Avenue du Bois de Boulogne; but I do not think he would be able to see the signal. By placing on the end of the searchlight a paper tube, the light would be invisible except in the direction in which it is pointed—and that, you will remember, is diagonally upward. A man on the sidewalk would not see it at all."
"Then, Monsieur, I fail to see that there is anything we can do."
"There is one thing, Vernet. You forget the answering signal, from the window."
The Frenchman looked at his companion with undisguised admiration. "Sacré!" he exclaimed. "You have a mind, Monsieur Duvall, in a thousand."
"Thanks," answered Duvall, dryly. "Now, my idea is, to have you select some point near the intersection of the two avenues, from which the window in the rear of Mr. Stapleton's house can be clearly seen. Station yourself there, tonight, with the fastest automobile you can secure. Let one man watch the window, another the vehicles passing in the Avenue du Bois de Boulogne. The moment you see the blue light, start after your man. He should be just across the intersection, on his way down the Avenue du Bois de Boulogne."
Vernet rubbed his hands together with satisfaction. "We shall get him—never fear."
"Of course," said Duvall, slowly, "all this is pure assumption on my part, based upon what I have discovered in the chauffeur's room. It may not turn out as I say, but the chances are fifty to one that it will."
"And you, Monsieur? Where will you be?"