i was at sporting goods store and owner had a peep sight or aperature sight that goes on your eye glasses.it has suction cup to hold it on.that was neat.he said he uses it to sight in his muzzleloader .. it is only about 1/2 inch big, unreal stuff they are coming out with.
This sounds like a Merit optical attachment.I have one and it does work great. I use it for target shooting but not hunting. It has an adjustable iris which you adjust and set so that your dominant eye looks though it as you sight your rifle. What is does is increase your depth of field so that both the front and rear sight, as well as the target, are all in focus as you sight the rifle. It was actually developed for pistol shooters but also works well with an open sighted rifle. If you want to see what it does, you can get the same effect by placing a small piece of electrical tape, with a small hole in the center,on your eyeglass lens in front of your dominant eye.