This article is the second in a series dealing with the hand-held instruments and techniques navigators have used to find distance.

Warships frequently steam in tight formation in order to concentrate defensive gunfire. It is the Conning Officer's responsibility to keep his ship on station at an assigned range and bearing from the guide ship, usually the flagship of the Senior Officer Present Afloat. This can be an easy task when the fleet is steaming along peacefully, but it becomes a particularly challenging responsibility when the formation is changing course frequently under battle conditions. In such difficult situations, the Conning Officer needs all the help he can get.

In the days before radar, and even now when radar and laser are blacked-out, the Conning Officer had to rely on optical instruments to determine range and bearing. Bearing always has been easy – a pelorus mounted on the wings of the bridge gives bearing quickly and accurately. Measuring range is more difficult. Many ingenious instruments relying on prisms or the doubly reflecting principles of the sextant have been produced to simplify the task of finding distance (Ifland, 2002).