Abstract

It has been reported that the human eye behaves as though relatively short-sighted in dim light. Observers tend to compensate for this change by setting optical instruments more negatively in dim than in bright light. New measurements of telescope settings by 21 observers reveal an average increase in power of the eye in dim light of 0.59 diopter (range +1.4 diopters to -3.4 diopters). The <i>dilation of the pupil</i> in dim light does not contribute significantly to this phenomenon. The <i>chromatic aberration of the eye</i> was measured in 14 observers with a specially designed <i>spectral stigmatoscope</i>. The refractive power of the eye increases about 3.2 diopters between 750 and 365 mµ. For this reason the Purkinje shift of maximum visual sensitivity from 560 mµ in bright light to 505 mµ in dim light produces a relative myopia in dim light of 0.35 to 0.40 diopter. Persons who display changes larger or smaller than this do so because of involuntary changes in the <i>accommodation</i> in bright and dim light. In dim light the eye enters a state of relatively <i>fixed focus</i>, little different from its condition when the accommodation is paralyzed with homatropine. In this fixed state the accommodation may be relaxed, or it may add as much as 3 diopters to the refractive power of the eye. Experienced observers focus optical instruments in dim light close to the optimal settings determined objectively. Departures of more than 0.5 diopter in either direction from the optimal focus depress the visual sensitivity and acuity. It is concluded that setting optical instruments about 0.4 diopter more negatively in dim than in bright light is justified on the basis of the chromatic aberration of the eye. Many observers gain a further advantage from individual adjustments of focus in dim light, appropriate to their accommodative behavior.

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