Theory
It was made up of a metallic head having four metal arms provided with two pairs of vertical slits giving two lines of sights at right offsets to each other at the center conical socket was provided which was mounted on the top of wooden staff. Filled with iron shoe when in use.
Use
1. Setting out right angle.
2. Finding foot of perpendicular.
Working principle of
If there are two plan mirrors whose reflected surface make a given angle with each other, if a given angle, if a ray of light undergoes two successive reflection from both of them, the angle between the first incident ray and last reflected ray is double the angle between two mirrors.
Diagram
Construction
It was made up of a gun metal hollow box with open face about 60 X 30 mm and close face about 20 X 30 mm. It was about 40 mm deep and was provided with a brass handle about 8 cm in-depth the mirror were fixed at the angle of 450 to the indicated sides of the box. Above the mirrors the opening in rectangular shapes where provided. The open face was directed towards the object of which the offsets was to be taken.
Use
Used for setting out right angle on the lines and finding foot of perpendicular from the object on the survey line.
