earlier posts, this was sometimes accomplished by constructing (probably using papier mâché) the convincing likeness of a rotting horse carcass. Hollowed out and equipped with peepholes, it was of sufficient size that a soldier could secretly listen during the day, then return to his unit in darkness.
It is impossible to know how often (if at all) "ye olde dead horse deception" was actually used on the battlefield. The only photographs we've seen were made as demonstrations at camouflage training camps, and distributed to various American news services, for amusing wartime anecdotes. If the method were actually used on the battlefield, it would not be wise to reveal it to the public.
Other devices were also employed, or at least they were tested at camouflage training camps. Reproduced above, for example, is a US Army photograph of three soldiers who were testing a curious listening device that consists of a mirrored sheet mounted on a wooden frame (tilted forward slightly). The irregularly-shaped leafy edge is an attempt to make it fit in better within a natural setting. Because the panel front is mirrored, a distant observer would see a reflected image of the terrain, but not the soldier behind it. It may seem like a brilliant idea, but it can't have been very effective. In this photograph, the three soldiers are positioned so closely together that their own bodies would be partly visible in the mirror of the person behind them.
The National Archives and Records Administration online documentation of this (NARA 55162019 and 55162023) includes an additional photograph (as in before-and-after views). In that image (shown below), we see the enemy's point of view (more or less) and—voila!—there is not a trace of the hidden listeners (well, almost).
This idea was not unprecedented at the time of WWI, and it has since resurfaced frequently, probably independently of its older wartime function. As one of many examples, reproduced below is a patent drawing for a "Reflective Hunting Blind," invented c2008 by Kevin Pottmeyer and Chester Burdette and registered as US Patent 8579007 B2. It is currently available as the GhostBlind Waterfowl Blind.
It consists of four hinged panels, irregularly shaped along the top edge, with peepholes through which a hunter can look. The inner surface is covered with a disruptive camouflage pattern, while the side that faces out (as shown here) is a reflective mirror surface (tilted forward slightly). As a result, the inventors explain—
A game animal looking into the front of the blind from a distance therefore sees only a reflection of the terrain surrounding the blind, thereby making the blind substantially indiscernible from the surrounding terrain and effectively obscuring a hunter…