An eyeglass is an optical instrument required by the clockmaker to magnify the image of an object and bring it closer to the eye. Eyeglasses may consist of a round, tapered tube fitted with a lens, usually single but occasionally multiple, and shaped to be held in the eye socket; or they may be held to the eye by a band worn round the head. Eyeglasses are also made in hinged frames which can be fitted to spectacles as required, and old illustrations show eyeglasses held in bench stands over the work. Lenses of different powers of focal length are available, and two-, three-, or four-in. Eyeglasses are used when the work is to be viewed at those distances. A double eye glass will allow the object to be seen c. 1/4in. From the lens. Eyeglasses with a tube are internally finished in matt black to prevent disturbing reflections, and have a ventilation hole in the side to prevent condensation on the lens.