What if we had eyes in the side of our head, like fish? 180 degree vision?
Looking into the eyehole, one's vision is met with a mirror at a 45 degree angle, and in that line of sight, another hole to the side... therefore, it won't be obvious to anyone looking on, but the person peering into the spyglass actually will have the ability to see what is standing not in the direction they appear to be looking in front of them, but to their side.
Made in Austria.