Right back at the start of the blog we looked at an example of the MK I marching compass here. Compasses don’t come up too often so it has taken until now for me to add another to my collection, in this case a Prismatic MK III:This compass is not in the best condition as the top piece of glass with the scale on it is badly cracked, but it will do until I can find a better example to add to the collection. The prismatic compass is a radically different design, being far more complicated as can be seen when the two designs are placed side by side:In the case of this compass the main body is made of brass, painted black with a scale around the body picked out in white:The dial itself is made of mother of pearl to help make it easier to read in low light and this is floating in oil:The prism that gives its name to the prismatic compass is hinged and can be folded up onto the main dial of the compass:The prism helps the user make more accurate readings than with a standard marching compass. A small dial is fixed to the side to allow the rotating bezel to be secured into position:A thumb ring is fixed to the end of the compass:By pushing the thumb through the ring and making a fist, a stable platform can be made for taking accurate readings. The base of the compass is marked with details of model, date, manufacturer and the /|\ acceptance mark:
T.G. Co Ltd stands for Thomas Glausser, who made the compass.
On a safety note the MK III prismatic compass used radium for the luminous paint on the dials. Although this has gone brown it is still releasing gamma and beta radiation. The beta radiation is contained inside the case, but the gamma radiation leaches out. They are safe to collect, but avoid excessive handling and don’t carry them regularly in your pocket!
These compasses are widely faked in India so collectors should be wary when purchasing. This site here has some helpful hints on spotting fakes.