SM167      Spherometer     circa 1963     Light

  The spherometer is an instrument that uses the principle of the micrometer screw to measure the radius of curvature of a spherical surface.   This consists of a disk graduated in say 500 divisions, which is turned by a milled head above.   The screw is precision turned so as to advance, say 0.5 mm with every turn of the disk.   The blunt end of the screw is at the centre of three symmetrically placed fixed legs.   In use the four points are made to touch the surface to be measured before a reading is taken.   The instrument is zeroed on an optically flat plate and then the travel of the screw to touch a spherical surface, say a mirror or lens can be accurately measured.

  If h is the distance of the moveable point from the plane of the fixed points, c is the distance of the moveable point from the fixed points when in the same plane and d the diameter of the sphere, then it can be shown by a simple geometrical construction that:   

As c is a fixed distance and h can be accurately found, the diameter of the sphere can be determined.   Another version of the spherometer gives the power of the lens directly assuming a value for the refractive index of glass.