houldn't want to perform the experiment."

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 len