The document describes the components and uses of a prismatic compass. It details the cylindrical metal box, pivot, lifting pin and lever, spring brake, magnetic needle, graduated circle, prism, object vane, eye vane, sunglasses, and reflecting mirror that make up the prismatic compass. It then explains that the prismatic compass is a small, handheld instrument used for observing and rough work, and that its prismatic attachment contains a 45 degree reflecting prism to magnify the graduations and allow the user to take readings.

2. THE PRISMATIC COMPASS

3. COMPONENTS OF PRISMATIC COMPASS

4. COMPONENTS OF PRISMATIC COMPASS

5. CYLINDRICAL METAL BOX
• Diameter of 8cm to 12cm.
• Protect the compass.
• Forms entire casing or body of the
compass.
COMPONENTS OF PRISMATIC COMPASS

6. PIVOT
• Centre of the compass.
• Supports the magnetic needle over it.
LIFTING PIN AND LIFTING LEVER
• Below the sight vane.
• Lifting pin pressed when sight vane is folded.
• The magnetic needle lifted out from the pivot point with
the helps of lifting lever.
COMPONENTS OF PRISMATIC COMPASS

7. SPRING BRAKE OR BRAKE PIN
• To damp the oscillation of the needle.
• To bring it to rest quickly.
MAGNETIC NEEDLE
• Heart of the instrument.
• Measures angle of a line.
• Always pointed to North and South pole.
COMPONENTS OF PRISMATIC COMPASS

8. GRADUATED CIRLCLE OR RING
• Aluminium graduated circle marked with 0˚ and 360˚ to
measure all possible bearings of lines.
• The ring is graduated to half a degree.
PRISM
• Read graduations on ring and to take exact reading by
compass.
• Placed exactly opposite to object vane.
• The prism hole is protected by prism cap to protect it from
dust and moisture.
COMPONENTS OF PRISMATIC COMPASS

9. OBJECT VANE
• Diametrically opposite to the prism and eye vane.
• Carrying horse hair or black thin wire to sight object in
line.
EYE VANE
• A fine slit provided with eye hole at bottom to bisect the
object from the slit.
• Take reading simulatenously from the eye hole.
• Provided with prism and can be lifted up and down by the
stud to bisect the object of higher level.
COMPONENTS OF PRISMATIC COMPASS

10. SUNGLASSES
• Used when some luminous objects are to be bisected.
• Placed in front of the eye slit and in bunch of 3 or 4 shades
of different colors to give sharp picture of the objects only.
REFLECTING MIRROR
• Used to get image of an object located above or below the
instrument level while bisection.
• Placed exactly opposite to object vane.
• The prism hole is protected by prism cap to protect it from
dust and moisture.
• Placed on the object vane.
COMPONENTS OF PRISMATIC COMPASS

12. USES OF PRISMATIC COMPASS
• The prismatic compass is a small instrument which is held in the hand for observing, and is
there employed on the rougher classes of work.
• The prismatic attachment consists of a 45° reflecting prism with the eye and reading faces
made slightly convex so as to magnify the image of the graduations.
• The prism is carried on a mounting which can be moved up and down between slides
fixed on the outside of the case. The purpose of this up-and-down movement is to provide
an adjustment for focusing.
• The image of the graduations is seen through a small circular aperture in the prism
mounting, and 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 V cut, the vertical
wire and the station whose bearing is required are viewed in one line, the bearing is
directly read off the graduated arc at the point immediately underneath the vertical wire.
• The oblong mirror located in front of the forward vane slides up and down the vane, and is
hinged to fold flat over it or to rest inclined at any angle with it.

13. USES OF PRISMATIC COMPASS
• This mirror is used for solar observations, or
for viewing any very high object, and is not a
normal fitting to a compass.
• The two circular discs in front of the back
vane are dark glasses which can be swung in
front of the vane when solar observations are
being taken.