The Type FT36 rangefinder was one of the models adopted by the Royal Navy in the 20th century for determining the distance of other ships. It consists of a brass tube with two eyepieces in the middle and windows at each end of the long tube, though which objects are viewed. To use the instrument, the observer looks through the eyepieces at an object such as a ship. The image seen will be in two halves (one from each window). The observer adjusts the instrument until the images from the two sides coincide and then reads the range (distance) off the scale, which is viewed through the left eyepiece.

The brass tube is covered in canvas, in order to protect the user's hands when using the rangefinder in very low temperatures. It is contained in a fitted wooden case, which holds a number of accessories, including a rail mounting, a carrying sling, a harness, an accumulator in a leather case, two rubber face-pieces, a packet of quills, a chamois leather, a handbook and a lamp for illuminating the range-scale when used at night.