Biomedical subjects
Publications and source records attributed to O W Richards.
One-hundred seventy-five persons viewed test letters of 35 and 11% contrast at 0.1 and 0.01 fL chart luminance while lenses (+0.25 to -1.50 D in 0.25-D steps) were placed before their eyes (and glasses, if they wore them). With one or more of the minus lenses, only 9% of the subjects obtained improved acuity by one or more lines on the chart. The most common optimum lens power was -0.75 D.
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Visual acuity of persons aged 16 to 90 years was measured with Snellen letters of varied contrast at 10, 1, 0.1, and 0.01 fl chart luminances. Percentage losses of seeing with age were computed. At 10 fl luminance, perception of high- and medium-contrast letters has changed little at age 40, but about twice as much light is needed to see low-contrast letters as at age 20. By age 70, no 2-min-subtense (20/40) letters were seen at 0.01 fl luminance. The need of older people for increased lighting during indoor tasks and night driving is discussed.
Looking through optical instruments stimulates the eye to accommodate more than would be necessary for naked eye viewing; this increased near power is called instrument myopia. The amount (up to -5 diopters) varies with the observer and the conditions of viewing. Using microscopes produces -1.5 to -2 diopters of instrument myopia. More instrument myopia is found with low-power magnification, inexperienced users, and when focusing form minus blur to sharp image. Physiologically, the instrument myopia appears to be associated with an accommodative equilibrium position and, psychologically, with a sensing of proximity.