I nodded. So far it was absurdly simple and clear. I was a trifle mortified that I had not divined it myself, but then such things were not in my line and the affair stuck too close to home to leave me any capacity for ratiocination.
"The question that had to be settled then," he continued, "was the situation of this entrance. I called your attention to the peculiar architecture of the house. When I entered the study I noticed that the safe occupied the wall in question. Jenkins opened it for me and I saw that it was the size of an ordinary closet and not very deep. What was more reasonable than to deduce that the remaining space between the back of the safe and the outer wall of the house was occupied by a passage of some kind!"
Again I nodded. "Of course. It was just a question of accounting for the extra square footage of house. But you haven't answered my original query."
"About the light? Mrs. Darwin said she didn't touch it, the dead man presumably couldn't, therefore the murderer must have done so. If he had pulled the cord Mrs. Darwin would have seen him, hence he lighted the lamp from some other source. Where? Not at the main switch near the door, for he had to vanish at once, knowing the shot would rouse the household. Besides, Mrs. Darwin would have heard the click when he pushed the button. The only place left was somewhere near the entrance. It was more likely to be inside than out, since, as before, Mrs. Darwin heard no sound. So I looked for it in the most plausible spot and found it."
I smiled. "You have answered my third question, which related to the secret entrance, but I have thought of two more to take its place. If the murderer used Darwin's pistol, how is it that only Ruth's finger-prints are on it?"
"He'd be too clever not to use gloves," returned McKelvie shortly.
"To be sure. But here's a harder one. How did the criminal, if he was behind Ruth, shoot Philip Darwin with such accuracy in the dark?"
"Exactly, that's just the point," he replied enigmatically.