In 1823, a new optical instrument began to appear in French opera houses that allowed patrons in the distant (and less expensive) seats to view the opera as if they were in the front row. Called opera glasses, the device combined telescope lenses with stereoscopic prisms to provide a magnified, three-dimensional view. After many years (but relatively few modifications), opera glasses have evolved into the binocular.

In their simplest form, binoculars are a pair of small refracting telescope lenses, one for each eye. The brain assembles the two views, one from each lens, into a single picture. Because each eye sees its own view, the final image has depth; this is not so with conventional telescopes, which possess only one eyepiece and, therefore, a two-dimensional image.

While some simple binoculars can be found, most quality binoculars possess a more intricate design. In more complex binoculars, there is a...