The Merit eyeglass aperture attaches to your eyeglasses or shooting glasses and has an adjustable peep hole. Turn the outside and the hole gets larger or smaller. It allows you to fine tune your sight picture with open sights for the best clarity and visibility in all brightness conditions. It works exactly like a camera aperture or the pupil of your eye. Using no lens, the Merit adjusts the size of its hole or aperture you're looking through, changing the amount of light admitted to lengthen or shorten the depth of field. You can actually see your sight picture sharpen up as you turn the adjustment ring. With practice you will be able to aim more and more precisely... It's amazing! This Merit Eyeglass Aperture works for iron sights on most rifle and handguns. If you're using a rifle with a Williams WGRS peep sight or EABCO PeepRib peep sight, you should order the Merit Adjustable Aperture for WGRS sights. See all of our TC muzzle loader accessories at our Muzzle Loader Home Page.