Using a prismatic compass first entails selecting the bearing and opening its lid, then folding it flat. Next, undo the curled knob, and move the bearing to the top. This is followed by tightening the knurled knob and lining up the two illuminated sticks. Finally, move the compass, and point it forward, ensuring that the two illuminated sticks do not separate.
The prismatic compass was invented by Charles Schmalcalder and patented in 1812. The graduations on the compass are located on a light aluminum ring fastened to a needle such that the zero point on the graduation scale coincides with the south point of the needle. When the compass is in use, the graduations remain unmoved by the needle while the index turns with the sitting vanes.
The prismatic attachment consists of a 45-degree reflecting prism with a slightly convex eye and reading faces that magnify the image of the graduations for easy reading. This image is seen through a small circular aperture in the prism mounting; immediately above this aperture is a small V cut on top of the mounting, over which the vertical wire in the front vane may be viewed. When the vertical wire, the V cut and the station whose bearing is required are viewed in one line, the bearing can then directly read off the graduated arc at the point immediately underneath the vertical wire.