I hate the sight of cards now.

I have seen so much of the ruin they do, and have, I am sorry to say, taken a hand so often in doing it, that save showing someone who would use the knowledge in the right way how the tricks are done, nothing would persuade me to touch them again.
However, as a protection, the knowledge is as useful as it is dangerous when used the other way.

It would take you ten years to learn to do these tricks yourself so well as to defy detection; but in a very short time, by learning where to keep your eyes, you would get to detect almost any of them.
"You see, there are three methods of cheating: the first by hidden cards, the second by marked cards, the third simply by sleight of hand, this being generally used in connection with marked cards.

These tricks require great skill and extreme delicacy of touch, for the marks, which are generally at the edge of the cards, are so slight as to be altogether imperceptible save to a trained hand.

There are also marks on the back of the cards; these are done in the printing, and are so slight that, unless attention were attracted to them, no one would dream of their existence." In the course of a week's practice Mark learned where to look for cheating; he could not indeed follow the fingers of his instructor, for even when he knew what was going to be done, the movements were so rapid that his eye could not follow them, and in nine cases out of ten he was unable to say whether the coup had been accomplished or not; but he could see that there was a slight movement of the fingers that could only mean that something was being done.
"It would be a good thing," he said one day, "if every young fellow before going out into the world were to have a course of such instruction as you are giving me; he would learn, at least, the absolute folly of sitting down to play cards with strangers.