That will anticipate any suggestion that the book has been tampered with after leaving your hands." "It would be a great impertinence for anyone to make any such suggestion," Mrs.Hornby began; but on Thorndyke's placing his fountain pen in her hand, she wrote her signature in the place indicated and handed the pen to Miss Gibson, who signed underneath.
"And now," said Thorndyke, "we will take an enlarged photograph of this page with the thumb-mark; not that it is necessary that it should be done now, as you are leaving the book in my possession; but the photograph will be wanted, and as my man is expecting us and has the apparatus ready, we may as well despatch the business at once." To this both the ladies readily agreed (being, in fact, devoured by curiosity with regard to my colleague's premises), and we accordingly proceeded to invade the set of rooms on the floor above, over which the ingenious Polton was accustomed to reign in solitary grandeur.
It was my first visit to these mysterious regions, and I looked about me with as much curiosity as did the two ladies.

The first room that we entered was apparently the workshop, for it contained a small woodworker's bench, a lathe, a bench for metal work and a number of mechanical appliances which I was not then able to examine; but I noticed that the entire place presented to the eye a most unworkmanlike neatness, a circumstance that did not escape Thorndyke's observation, for his face relaxed into a grim smile as his eye travelled over the bare benches and the clean-swept floor.
From this room we entered the laboratory, a large apartment, one side of which was given up to chemical research, as was shown by the shelves of reagents that covered the wall, and the flasks, retorts and other apparatus that were arranged on the bench, like ornaments on a drawing-room mantelpiece.

On the opposite side of the room was a large, massively-constructed copying camera, the front of which, carrying the lens, was fixed, and an easel or copyholder travelled on parallel guides towards, or away, from it, on a long stand.
This apparatus Thorndyke proceeded to explain to our visitors while Polton was fixing the "Thumbograph" in a holder attached to the easel.
"You see," he said, in answer to a question from Miss Gibson, "I have a good deal to do with signatures, cheques and disputed documents of various kinds.

Now a skilled eye, aided by a pocket-lens, can make out very minute details on a cheque or bank-note; but it is not possible to lend one's skilled eye to a judge or juryman, so that it is often very convenient to be able to hand them a photograph in which the magnification is already done, which they can compare with the original.
Small things, when magnified, develop quite unexpected characters; for instance, you have handled a good many postage stamps, I suppose, but have you ever noticed the little white spots in the upper corner of a penny stamp, or even the difference in the foliage on the two sides of the wreath ?" Miss Gibson admitted that she had not.
"Very few people have, I suppose, excepting stamp-collectors," continued Thorndyke; "but now just glance at this and you will find these unnoticed details forced upon your attention." As he spoke, he handed her a photograph, which he had taken from a drawer, showing a penny stamp enlarged to a length of eight inches.
While the ladies were marvelling over this production, Polton proceeded with his work.

The "Thumbograph" having been fixed in position, the light from a powerful incandescent gas lamp, fitted with a parabolic reflector, was concentrated on it, and the camera racked out to its proper distance.
"What are those figures intended to show ?" inquired Miss Gibson, indicating the graduation on the side of one of the guides.
"They show the amount of magnification or reduction," Thorndyke explained.