The prism on the side allows the viewer to also take bearings when using the tall folding point sight. It has the number 678 engraved on the top. The compass is based on the prismatic compass by Charles Schmalcalder (British patent in 1812), which lets the user read the card while sighting a distant object. It was termed an Improved Hutchinson Prismatic Compass, said to be lighter and less bulky'than the old form' and was adopted by the various branches of the British and Foreign Services.Size: 10 x 7 x 2.4 cm (4 x 2 ¾ x 1 inches) Weight: 188 g. Have a look at our other items. If you have any questions we are more than happy to help. _gsrx_vers_841 GS 7.0.16 (841). The item "C1875 Pocket Prismatic Compass by J Hicks London" is in sale since Thursday, January 31, 2019. This item is in the category "Antiques\Science/Medicine\Scientific Instruments". The seller is "paradeantiques_online" and is located in Plymouth. This item can be shipped worldwide.