"They were washed last Friday, but they do look rather dirty, don't they? Suppose you take a rag and some scouring soap and clean up a pane."
The detective took the proffered articles and started his task. He wet a pane of glass, rubbed up a thick lather of scouring soap and applied it and rubbed vigorously. With clear water he washed the glass and then gave an exclamation of astonishment and examined it more closely.
"That isn't dirt, Doctor," he cried. "The glass seems to be fogged."
Dr. Bird chuckled.
"So it seems," he admitted. "Now look at the rest of the glass around the laboratory."
Carnes looked around and then walked to a table littered with apparatus and examined a dozen pieces carefully.
"It's all fogged in exactly the same way, Doctor," he said. "The only piece of clear glass in the room is that piece of plate glass on your desk."
r. Bird picked up a hammer and struck the plate on his desk a sharp blow. Carnes ducked instinctively, but the hammer rebounded harmlessly from the plate.