He drew from the portfolio a half-plate photograph of the thumb-print which appeared magnified to about four inches in length.  The print was marked by a number of figures written minutely with a fine-pointed pen, each figure being placed on an “island,” a loop, a bifurcation or some other striking and characteristic portion of the ridge-pattern.

“This system of marking with reference numbers,” said Mr. Singleton, “is better than your method of squares, because the numbers are only placed at points which are important for comparison, whereas your squares or the intersections of the lines fall arbitrarily on important or unimportant points according to chance.  Besides, we can’t let you mark our original, you know, though, of course, we can give you a photograph, which will do as well.”

“I was going to ask you to let me take a photograph presently,” said Thorndyke.

“Certainly,” replied Mr. Singleton, “if you would rather have one of your own taking.  I know you don’t care to take anything on trust.  And now I must get on with my work, if you will excuse me.  Inspector Johnson will give you any assistance you may require.”

“And see that I don’t pocket the original,” added Thorndyke, with a smile at the inspector who had shown us in.

“Oh, I’ll see to that,” said the latter, grinning; and, as Mr. Singleton returned to his table, Thorndyke unlocked the microscope case and drew forth the instrument.

“What, are you going to put it under the microscope?” exclaimed Mr. Singleton, looking round with a broad smile.

“Must do something for my fee, you know,” replied Thorndyke, as he set up the microscope and screwed on two extra objectives to the triple nose-piece.  “You observe that there is no deception,” he added to the inspector, as he took the paper from Mr. Singleton’s table and placed it between two slips of glass.

“I’m watching you, sir,” replied the officer, with a chuckle; and he did watch, with close attention and great interest, while Thorndyke laid the glass slips on the microscope stage and proceeded to focus.

I also watched, and was a good deal exercised in my mind by my colleague’s proceedings.  After a preliminary glance with the six-inch glass, he swung round the nose-piece to the half-inch objective and slipped in a more powerful eye-piece, and with this power he examined the blood-stains carefully, and then moved the thumb-print into the field of vision.  After looking at this for some time with deep attention, he drew from the case a tiny spirit lamp which was evidently filled with an alcoholic solution of some sodium salt, for when he lit it I recognised the characteristic yellow sodium flame.  Then he replaced one of the objectives by a spectroscopic attachment, and having placed the little lamp close to the microscope mirror, adjusted the spectroscope.  Evidently my friend was fixing the position of the “D” line (or sodium line) in the spectrum.