Often the refractionist finds the ophthalmometer value varying significantly from that of the proper correcting cylinder of the spectacle lens. Many still use the rule of Javal1 to modify the ophthalmometer reading to approximate the proper cylindrical value of the correcting spectacle lens. In an article2 published by the American Optical Company the rule of Javal is found faulty. The problem is analyzed, and, in a series of algebraic gymnastics, a formula is derived upon which are based the data in the Table, i. e., the upper figures in each row. These data represent corrected cylindrical components as derived from the influence of the spherical component upon the ophthalmometer reading and are for a vertex distance of 13.75 mm.
Neither the data of the Table nor the formula upon which they are based is easily committed to memory. The "Rule of 7 or 14 Thirds" is a useful
NADBATH RP. The Rule of Seven or Fourteen Thirds. AMA Arch Ophthalmol. 1958;60(4):534–536. doi:10.1001/archopht.1958.00940080552002