Watermarks in paper only appear with transmitted light (light going through). Here is a Crane Bros. watermark from 1875 found in a letter from Staniar & Laffey, manufacturers of dandy rolls which are used to make watermarks on Fourdrinier paper machines.
This sketch was used for the creation of the dandy roll with the Crane Bros. watermark.
Here is the full letter with backlighting so the watermark is visible.
Here is the same letter without backlighting so the watermark is invisible.