“ The principle of the vernier is readily understood from the following account: Let AB (see fig.) be the normal scale, i.e. a scale graduated according to a standard of length, CD, a scale (placed in contact with AB for convenience) graduated so that 10 divisions equal 11 divisions of the scale AB, and EF a scale placed similarly and graduated so that 10 divisions equal 9 divisions of the scale AB. Consider the combination AB and CD. Obviously each division of CD is 1/16th greater than the normal scale division. ”