The presbyopic add. I. Magnitude and distribution in a historical context.
The presbyopic addition or 'add' is the (binocular) spherical positive power supplement superimposed on any distance refractive correction to counter near vision difficulties associated with the physiological age-related loss of accommodation and consequent recession of the near point: its necessity is first encountered in middle age. The continuous (linear) ascending trend in near lens power versus increasing age advocated since the seventeenth century is at odds with the twentieth-century clinical observation that there is a deceleration in add requirement beyond an individual's mid-fifties. It is shown here that this difference is reconciled--in broadly qualitative terms if not in magnitude-when typical clinical near addition data are superimposed on the accepted distance hypermetropic refractive base recorded from early middle age onwards: a continuous rising function is produced spanning most of the three decades beyond 40 years of age. This result is considered in the context of the age markings found on certain eighteenth-century antique near vision glasses and the recommendations attached to the self-selection of ready-made reading glasses in the late twentieth century.