In the nineties of the last century was sold a curious instrument under the great name: “x-ray”. Remember how I was taken aback when a schoolboy he first picked up this ingenious fiction: tube gave the opportunity to literally see through opaque objects!
I discerned the surrounding not only through thick paper, but also through the blade of a knife, impenetrable even to the genuine x-rays. Simple secret devices of this toy immediately becomes clear if you look at the picture representing the prototype of the described tube. Four mirror, inclined at an angle of 45°, reflect rays several times, leading them, so to speak, bypassing the opaque object.
Imaginary x-ray machine.
In military Affairs widespread use of similar devices. Sitting in a trench, you can keep track of the enemy, without raising his head above the ground and, therefore, not putting themselves under the fire of the enemy, if you look in the device, called a “periscope”.
The longer the path of light rays from entering the periscope to the observer's eye, the narrower the field of view that is visible in the device. To increase the field of view, is a system of optical glasses. However, glass absorb part of the light penetrating through the periscope; clear visibility of the subject suffers from this. Said puts limitations to the height of the periscope; two dozen meters are already high, approaching the limit; higher periscopes give too small a field of view and indistinct images, especially in cloudy weather.
The captain of the submarine watching the attacked ship through the periscope - long tube, the end of which protrudes above the water. These periscopes are much harder than the land, but the essence is the same: the rays are reflected from a mirror (or prism), fortified in the protruding part of the periscope, go along the tube, is reflected in its lower part and get into the eye of the observer.
Diagram of periscope of a submarine.