This spherometer consists of two metal bars that are joined at one end to pivot a very short arc of movement that is measured at the opposite end. The center ball is fixed to one bar, the two feet balls to the other, all in a line. Since the center ball is located at one third the distance to the micrometer, it moves one third the travel as measured on the micrometer.
There is an additional bolt placed near the pivot bolt on one of the bars such that it lines up with one of the feet and will pinch the lens near the edge to measure wedge. Although the arc of movement creates a vector, the movement toward the lens is 10x that of the linear movement of the micrometer, resulting in tremendous sensitvity. The success of accurate measurements depends on a very light touch to sense when contact is made, the arms held together and the micrometer turned until it meets the end plate. Wedge error of over .0005" cannot be tolerated in the 7.125" flint element and I believe I achieved .0004"-.0003" with considerable confidence.
The bars are held under pressure by a light spring that keeps continuous tension on them at the pivot bolt, to eliminate any slop during the measuring movement.
The spherometer assembly is seated on a wooden jig that holds it at a 45 deg. angle from the horizontal, while the lens is seated at 90 deg to the bars and also at 45 deg, from the horizontal. This configuration ensures that the lens and spherometer are held in place by gravity and consistantly returned to their same positions relative to one another.
Advantages of this design are that it's cheap, quite sensitive, and will check wedge very effectively as well.
Disadvantages are that it requires a very light touch to avoid introducing significant flexure, is more time consumming to calculate the sensitivity amplification factor than a dial indicator setup and requires at least a metal cutting band saw to fabricate. It took me several full evenings in the shop to construct.