Question

give reason for three legs of spherometer

Answer 1

A spherometer in an instrument for the precise measurement of the radius of curvature of the sphere or a curved surface. Originally these instruments were used by opticians to measure the curvature of the surface of lenses.
In general, the spherometer consists of 
1. A base circle of three legs, the radius of the base circle is already known.
2. A central leg which van be raised or lowered.
3. A reading device for measuring the distance when the central leg is moved.

Answer 2

Spherometers have three legs that frame an equilateral triangle. Review from geometry that three focuses decide a plane, so the three legs will dependably touch the glass. In any case, you additionally know for a fact that four focuses don't, think about a flimsy bistro table. A fourth leg or point is situated at the middle the three legs. This essential issue is joined to a micrometer screw that can be raised or brought down. Micrometers are gadgets, for example, calipers that make an interpretation of little estimations into bigger developments of the gadget to permit exact estimations. One finish turn of the focal screw of the spherometer raises or brings down the middle point a set separation which is controlled by the measure of the string of the screw and the span of the plate. Every revolution of the screw raises the plate one unit on the vertical scale. To make estimations to two decimal places, the level plate of the spherometer is regularly set apart to offer readings to hundredths of a turn. The separation the main issue is above or underneath the plane made by the three legs is then increased by the length related with one turn of the micrometer. Along these lines a spherometer can gauge both an emphatically or contrarily bended surface to high exactness.