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 dilation of the pupil in dim light does not contribute significantly to this phenomenon. The chromatic aberration of the eye was measured in 14 observers with a specially designed spectral stigmatoscope. 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 accommodation in bright and dim light. In dim light the eye enters a state of relatively fixed focus, 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.

© 1947 Optical Society of America

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Figures (11)

F. 1
F. 1

Sectional view of spectral stigmatoscope. The lamp, filters, shutter, iris, and light point or “stigma” are mounted in a single housing which the observer moves back and forth by means of a rack and pinion until the stigma appears in sharp focus. Accommodation is stabilized by looking directly through the instrument at an external target (not shown). The light source is a G. E. H-3 mercury arc, from which the short ultraviolet radiation is screened with a Corning 738 filter, and spectral lines and narrow bands are isolated with monochromatic filters. The cornea of the observer’s eye is set at the principal focus of the achromatic lens by means of a movable head rest, which is adjusted so that the vertex of the cornea is in line with the corneal sight shown in the figure.

F. 2
F. 2

Axial chromatic aberration of the human eye. Averages of measurements on 14 observers. The ordinates show the lens correction in diopters needed to give each eye the same refractive power that it possesses when accommodated for distance vision at 578 mμ.

F. 3
F. 3

Axial chromatic aberration of the human eye. The present measurements compared with those of previous observers and with the calculated chromatic aberration of a hypothetical eye composed entirely of water. Ordinates as in Fig. 2.

F. 4
F. 4

Log brightness needed to achieve a constant visual acuity at various focus settings of a 7×50 telescope. Observer D. G. Accommodation paralyzed with 2 drops of 3 percent homatropine; near point, 1 meter. Observer’s own focus settings in dim and bright light for the normal and homatropinized eye are indicated with arrows.

F. 5
F. 5

Log brightness thresholds for constant visual acuity at various focus settings of a 7×50 telescope. Observer T. H. Accommodation paralyzed with 2 drops of 3 percent homatropine; near point, 60 cm. Observer’s own focus settings are indicated with arrows.

F. 6
F. 6

Log brightness thresholds for constant visual acuity at various focus settings of a 7×50 telescope. Observer M. L. Accommodation paralyzed with 2 drops of 3 percent homatropine; near point, 64 cm. Observer’s own focus settings are indicated with arrows.

F. 7
F. 7

Log brightness thresholds for constant visual acuity at various focus settings of a 7×50 telescope. Observer T. C. Accommodation paralyzed with 3 drops of 1 percent paradrene plus 2 drops of 3 percent homatropine; near point, 1.5 to 2 meters. Observer’s own focus settings are indicated with arrows.

F. 8
F. 8

Log brightness thresholds for constant visual acuity at various focus settings of a 7×50 telescope. Observer A. S. Accommodation paralyzed with 3 drops of 1 percent paradrene plus 1 1 2 drops of 3 percent homatropine; near point greater than 1 meter. Observer’s own focus settings are indicated with arrows. This is a hyperopic subject who accommodates habitually to achieve a high visual acuity without spectacles.

F. 9
F. 9

Effect of off-setting of a telescope on the visual acuity in bright daylight. Ordinates: number of lines of a Snellen chart read at 290 feet. When the telescope is offset in the negative direction, the normal eye maintains its visual acuity by compensatory accommodation. After treatment with homatropine (4 drops 5 percent) this capacity is lost, and the visual acuity declines sharply with off-setting of the focus in either direction.

F. 10
F. 10

Log brightness for just seeing a small source of monochromatic light when in and out of focus. Wavelength 621 mμ. Abscissae in diopters, as though the source were at a fixed distance, and lenses of the stated powers were placed before the eye. The measurements show that the visual threshold for a small source of light is minimal when it is in exact focus; and that displacements of focus raise the threshold much more in the fovea than in the peripheral retina.

F. 11
F. 11

Log brightness for just seeing a small source of monochromatic light when in and out of focus. Wavelength 546 mμ. Otherwise as in Fig. 10.

Tables (2)

Tables Icon

Table I Changes in focus setting in bright and dim light. Data of 8 observers, using binoculars of various types under various conditions. The table lists the differences between average settings in bright and in dim light in diopters. A minus sign indicates a shift to more negative settings in dim light.

Tables Icon

Table II Axial chromatic aberration of the human eye. Measurements on 14 observers, with the spectral stigmatoscope. The axial aberration is expressed as the lens correction in diopters required at each wave-length to bring the eye to the same power it possesses at 578 mμ. The actual settings for each observer at 578 mμ are shown in the fourth column; all other settings are stated relative to these. The “water eye” is a structure of the same posterior focal length as the human eye, but entirely composed of distilled water, and possessing a single refracting surface.