Prismatic Compass

This Prismatic Compass is one of the two main kinds of magnetic compasses  included in the collection for the purpose of measuring magnetic bearings, with the  other being the The main difference between the two instruments is that the  surveyor's compass is usually the larger and more accurate instrument, and is  generally used on a stand or tripod. The prismatic compass on the other hand is  often a small instrument which is held in the hand for observing, and is therefore  employed on the rougher classes of work.
The graduations on this prismatic  compass are situated on a light aluminium ring fastened to the needle, and the zero  of the graduations coincides with the south point of the needle. The graduations  therefore remain stationary with the needle, and the index turns with the sighting  vanes. Since the circle is read at the observer's (rather than the target's) end, the  graduations run clockwise from the south end of the needle (0° to 360°), whereas in  the surveyor's compass, the graduations run anti‐clockwise from north.