Cust's Patent Rangefinder (Museum object NAV1581) was designed to be mounted on an Officer of the Watch telescope or on a monocular. It works from knowing the height of a masthead, funnel or other feature of a ship, with the distance measured from the angle subtended by that feature. When placed in front of the telescope, two prisms in the rangefinder produce a double image, which is altered by revolving the front prism. When the double image is resolved into a single one, the angle is read off the graduated scale, which has a vernier allowing it to be read to an accuracy of 10 minutes. The angle can then be converted into a height using a slide rule that was supplied with the rangefinder. This example is inscribed with the name of the manufacturer and with its patent number (no. 15221). It is contained in a leather carrying case.
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Cust demonstrating his Patent Rangefinder, circa 1904 appears in: