I encountered a very clever device in a museum that was said to bestow users with a sense of what it was like to be dyslexic. You had to try to write your name in front of a mirror so that the reflection of the letters looked right. It is extremely frustrating.
Try it with a regular mirror. Hold a pad of paper right up to a mirror and write so that the letters read normally in the mirror. The periscope does allow you to read normally because the second mirror un-does what the first mirror did.
Did you know that when you look in a mirror, you don't see yourself as others see you? The left and the right of you are reversed. So why doesn't the mirror reverse up and down? I don't know myself.
If you take even a small mirror and use it to reflect the sun onto things a hundred feet or more distant, you get a sense of just how bright the sun is. No flashlight would be able to do that. In some countries where fuel is in short supply, some people use curved mirrors to concentrate reflected sunlight enough to cook with. It is said that a bunch of ancient Greek warriors used polished metal shields to start a fire on an enemy ship by all aiming the reflected sunlight at the same spot.