Daily fluctuations in the detectability of dim lights by humans

Several aspects of retinal physiology are known to vary on daily or circadian cycles, suggesting that certain aspects of visual function might also vary. We measured absolute threshold--the dimmest visual stimulus that can be reliably detected--in seven human observers at mid-day (1200-1400) and mid-night (0000-0200), by means of standard psychophysical techniques. Five of the subjects were slightly more sensitive at night: they could detect stimuli that were on the average 20% dimmer at night than during the day, and this difference was not due to changes in the criterion for detection. Three of four subjects whose thresholds were measured hourly while they remained otherwise in total darkness for greater than 24 hours showed changes in absolute visual threshold that correlated with hourly changes in body temperature. These results suggest that the ability to detect dim lights varies systematically with time of day in many human observers.

A schematic eye for the mouse, and comparisons with the rat

The thicknesses and spheric and aspheric curvatures of the optic components were measured from cross-sections of frozen eyes of C57B1/6J mice. The equivalent refractive index of the crystalline lens was obtained from its back-vertex power in albumin. Refractive indices of the cornea, aqueous and vitreous humors were obtained by refractometry or interferometry at four wavelengths across the visible spectrum. The measurements parallel earlier ones on the hooded rat. The eyes of the mouse and rat differ mainly in size, by a linear scale factor of 1.9-2.0, and only slightly in refractive index. Thus refraction, chromatic aberration, and retinal illumination are easily compared in the two species. An analysis of the contribution of each optical surface to refraction may facilitate extrapolation to other strains of mice. Chromatic aberration is discussed with respect to depth of field and the retinoscopy artefact.

EFFECT OF MELANOPHORE-STIMULATING HORMONE ON HUMAN DARK ADAPTATION

A preparation of melanophore-stimulating hormone (MSH) obtained from porcine anterior pituitary glands was found to produce no effect on the course of dark adaptation in normal human subjects, when injected subcutaneously in doses of 185 or 550 I.U.A partially purified MSH was prepared from bovine posterior pituitary glands by Hanaoka's modification of Stehle's method. It was assayed by a skin reflectance method in hypophysectomized frogs. When administered subcutaneously in a dose of 500 I.U., this material also failed to affect the course of dark adaptation in six normal subjects of Caucasian ancestry and six of Japanese ancestry.These results are compared with those of other investigators, and the disagreements are attributed to differences in apparatus and technique of measurement of dark adaptation. It is concluded that exogenous MSH does not affect dark adaptation in normal subjects.