He looked at his watch, and, as we rose from the table, he said: 

“If we have finished, we had better go up to the laboratory and see that the apparatus is ready.  Mr. Britton is a busy man, and, as he is doing us a great service, we mustn’t keep him waiting when he comes.”

We ascended to the laboratory, where Polton was already busy inspecting the massively built copying camera which—­with the long, steel guides on which the easel or copy-holder travelled—­took up the whole length of the room on the side opposite to that occupied by the chemical bench.  As I was to be inducted into the photographic art, I looked at it with more attention than I had ever done before.

“We’ve made some improvements since you were here last, sir,” said Polton, who was delicately lubricating the steel guides.  “We’ve fitted these steel runners instead of the blackleaded wooden ones that we used to have.  And we’ve made two scales instead of one.  Hallo!  That’s the downstairs bell.  Shall I go sir?”

“Perhaps you’d better,” said Thorndyke.  “It may not be Mr. Britton, and I don’t want to be caught and delayed just now.”

However, it was Mr. Britton; a breezy alert-looking middle-aged man, who came in escorted by Polton and shook our hands cordially, having been previously warned of my presence.  He carried a small but solid hand-bag, to which he clung tenaciously up to the very moment when its contents were required for use.

“So that is the camera,” said he, running an inquisitive eye over the instrument.  “Very fine one, too; I am a bit of a photographer myself.  What is that graduation on the side-bar?”

“Those are the scales,” replied Thorndyke, “that shows the degree of magnification or reduction.  The pointer is fixed to the easel and travels with it, of course, showing the exact size of the photograph.  When the pointer is opposite o the photograph will be identical in size with the object photographed; when it points to, say, x 6, the photograph will be six times as long as the object, or magnified thirty-six times superficially, whereas if the pointer is at / 6, the photograph will be a sixth of the length of the object, or one thirty-sixth superficial.”

“Why are there two scales?” Mr. Britton asked.

“There is a separate scale for each of the two lenses that we principally use.  For great magnification or reduction a lens of comparatively short focus must be used, but, as a long-focus lens gives a more perfect image, we use one of very long focus—­thirty-six inches—­for copying the same size or for slight magnification or reduction.”

“Are you going to magnify these cheques?” Mr. Britton asked.

“Not in the first place,” replied Thorndyke.  “For convenience and speed I am going to photograph them half-size, so that six cheques will go on one whole plate.  Afterwards we can enlarge from the negatives as much as we like.  But we should probably enlarge only the signatures in any case.”