Continued from here:
The Red Headed League, from The Adventures Of Sherlock Holmes.
In this story, Jabez Wilson visits Baker Street and consults Holmes on (yet another) singular problem. During their initial interview Holmes does what Holmes does and deduces all sorts of things about Wilson. It’s an amazing piece of Sherlockiana even by Holmes’s usual amazing standards. Wilson, is thoroughly flabbergasted and wonders how Holmes could possibly know all the stuff he comes out with. For the benefit of Watson—and readers—Holmes explains.
When Holmes has finished:
Mr. Jabez Wilson laughed heavily. "Well, I never!" said he. "I thought at first that you had done something clever, but I see that there was nothing in it, after all."
We then get:
"I begin to think, Watson," said Holmes, "that I make a mistake in explaining. ‘Omne ignotum pro magnifico,’ you know, and my poor little reputation, such as it is, will suffer shipwreck if I am so candid."
Exactly the same with magic tricks; "Oh is that all it is".