I came across this lovely object on the web and learned that it was a pair of 19th C. shooting spectacles. What, I wondered, are shooting spectacles? It turns out they are an important aid to marksmanship and still in use today.
A marksman sights on a target with his or her dominant eye but, for best results, both eyes should be open. There are two reasons: shutting one eye affects the amount of light available for vision; and keeping that eye shut over a long period (such as a championship match) will become tiring. Meanwhile, a shooter will want to train the dominant eye to sight properly on a target. So a good set of shooting glasses will block the vision of the non-dominant eye but still allow light to reach it and have a small opening or even an adjustable iris on the dominant eye lens so that focus can be narrowed. The Victorian version uses smoked glass in the non-dominant lens, modern versions use a translucent guard. The iris for the dominant eye hasn’t changed much except that the modern version is set into a metal backing rather than a wooden one.
Air gun shooting is an Olympic event and contestants use highly developed, specialized equipment. Shooting spectacles have become shooting glasses but there seems little change in the design or capability of these instruments.