“Major Sholto was a very particular friend of papa’s,” she said. “His letters were full of allusions to the major. He and papa were in command of the troops at the Andaman Islands, so they were thrown a great deal together. By the way, a curious paper was found in papa’s desk which no one could understand. I don’t suppose that it is of the slightest importance, but I thought you might care to see it, so I brought it with me. It is here.”

Holmes unfolded the paper carefully and smoothed it out upon his knee. He then very methodically examined it all over with his double lens.

“It is paper of native Indian manufacture,” he remarked. “It has at some time been pinned to a board. The diagram upon it appears to be a plan of part of a large building with numerous halls, corridors, and passages. At one point is a small cross done in red ink, and above it is ‘3.37 from left,’ in faded pencil-writing. In the left-hand corner is a curious hieroglyphic like four crosses in a line with their arms touching. Beside it is written, in very rough and coarse characters, ‘The sign of the four,—Jonathan Small, Mahomet Singh, Abdullah Khan, Dost Akbar.’ No, I confess that I do not see how this bears upon the matter. Yet it is evidently a document of importance. It has been kept carefully in a pocket-book; for the one side is as clean as the other.”

“It was in his pocket-book that we found it.”

“Preserve it carefully, then, Miss Morstan, for it may prove to be of use to us. I begin to suspect that this matter may turn out to be much deeper and more subtle than I at first supposed. I must reconsider my ideas.” He leaned back in the cab, and I could see by his drawn brow and his vacant eye that he was thinking intently. Miss Morstan and I chatted in an undertone about our present expedition and its possible outcome, but our companion maintained his impenetrable reserve until the end of our journey.