Hand held refracting telescopes are categorized by the number of retractable tubes they employ. Thus an instrument may be a single-draw, two-draw, three-draw and so forth. They are further categorized by the way lenses are arranged in the tubes as mentioned above. Very early telescopes were made of cardboard or wood covered with shagreen, vellum or leather. Bone, ivory or wood rings were used to hold lenses. From the 1750s to mid-1800s most telescopes were made of brass tubes with the front barrel encased in walnut, mahogany or fruitwood. Most telescopes made from the mid-1800s well into the 1900s were made entirely of brass with leather covers common on the front barrel. In the early to mid 1800s some naval telescopes, inscribed with ‘Day or Night’ on the drawtube, were designed for night and low light conditions. A three-draw telescope with red leather cover and gilt tooling, inscribed ‘1661’, is one of the oldest known surviving optical instruments and housed at the Maritime Museum in England.