The Collaborative International Dictionary
Prismatic \Pris*mat"ic\, Prismatical \Pris*mat"ic*al\, a. [Cf. F. prismatique.]
Resembling, or pertaining to, a prism; as, a prismatic form or cleavage.
Separated or distributed by a prism; formed by a prism; as, prismatic colors.
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(Crystallog.) Same as Orthorhombic.
Prismatic borax (Chem.), borax crystallized in the form of oblique prisms, with ten molecules of water; -- distinguished from octahedral borax.
Prismatic colors (Opt.), the seven colors into which light is resolved when passed through a prism; primary colors. See Primary colors, under Color.
Prismatic compass (Surv.), a compass having a prism for viewing a distant object and the compass card at the same time.
Prismatic spectrum (Opt.), the spectrum produced by the passage of light through a prism.
Wikipedia
A prismatic compas is a navigation and surveying instrument which is extensively used for determining course, waypoints (an endpoint of the leg of a course) and direction, and for calculating bearings of survey lines and included angles between them. Compass surveying is a type of surveying in which the directions of surveying lines are determined with a magnetic compass, and the length of the surveying lines are measured with a tape or chain or laser range finder. The compass is generally used to run a traverse line. The compass calculates bearings of lines with respect to magnetic north. The included angles can then be calculated using suitable formulas in case of clockwise and anti-clockwise traverse respectively. For each survey line in the traverse, surveyors take two bearings that is fore bearing and back bearing which should exactly differ by 180 if local attraction is negligible. The name Prismatic compass is given to it because it essentially consists of a prism which is used for taking observations more accurately.
Usage examples of "prismatic compass".
The main elements in our belt kit would be ammunition, water, emergency food, survival kit, shell dressings, a knife, and a prismatic compass as a backup for the Silva compass and for taking a bearing off the ground.
Here Pengelly, accompanied by two of the hands, went ashore, the new captain taking with him a prismatic compass.
She had watched closely as Zouga plotted a course with the prismatic compass, and she had mastered the technique of sighting a distant hill or other feature and marching upon it, before again sighting ahead on another feature.
Then Harvey gave him a prismatic compass that was designed as an old-fashioned turnip watch (complete with a chain that was used as a measure for distance-judging).