Coincidence rangefinder

A coincidence rangefinder is like playing "I Spy" with your eyes. When you're playing "I Spy," you have to find a certain object among many other objects in the room. For example, someone might say "I spy with my little eye something that is blue," and you'd have to look around the room to find something that is blue.

A coincidence rangefinder does something similar, but instead of finding a blue object, it helps you find the distance of something far away. It works by using two different parts of your eyes to look at the same thing at the same time. Just like how you might use both of your eyes to look at something really far away, like a bird flying in the sky, the coincidence rangefinder uses two lenses to focus on a distant object.

The coincidence rangefinder works by moving the lenses until the two parts of the object you're looking at line up perfectly. When the two parts of the object line up, that means they are the same distance away from your eyes. The coincidence rangefinder can then use some math and a ruler inside the device to figure out exactly how far away that object is.

So, to sum it up, a coincidence rangefinder uses the same idea as "I Spy" to help you figure out how far away something is by getting both parts of your eyes to look at it at the same time and measuring the distance between them.