I think the novel explained it thusly...

Posted by graml from user-2iveusr.dialup.mindspring.com on June 28, 2001 at 13:37:53:

In Reply to: Was it an optical illusion? posted by Jay on June 28, 2001 at 13:31:32:

The bridge blended into the pattern of the stone across the gap. And it was set about a foot or so below the lip of the ledge, so a person wouldn't notice it. Therefore, the bridge really did require a person to rely on his faith. But before I read the novel, I always assumed that it was just an invisible bridge. That's not too far fetched, is it? ;)
-graml

: When he threw the pebbles over the bridge, it looked like it was invisible instead of an optical illusion.

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