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 replied, "and then—well, Miller's suggestion wasn't such a bad one; to explore the surface with the microscope."

"I think we will take the latter measure first," said he. "The tram-ticket might create a misleading bias. A man may take a tram anywhere, whereas the indoor dust on a man's coat appertains mostly to a definite locality."

"Yes," I replied; "but the information that it yields is excessively vague."

"That is true," he agreed, taking up the coat and envelope to carry them to the laboratory, "and yet, you know, Jervis, as I have often pointed out, the evidential value of dust is apt to be under-estimated. The naked-eye appearances—which are the normal appearances—are misleading. Gather the dust, say, from a table-top, and what have you? A fine powder of a characterless grey, just like any other dust from any other table-top. But, under the microscope, this grey powder is resolved into recognizable fragments of definite substances, which fragments may often be traced with certainty to the masses from which they have been detached. But you know all this as well as I do."

"I quite appreciate the value of dust as evidence in certain circumstances," I replied, "but surely the information that could be gathered from dust on the coat of an unknown man must be