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I Rod and Cone AdaptationHuman dark adaptation, though described by Aubert in 1865 and first measured by Piper in 1903, is still inadequately known. The early data of Piper seemed to show that dark adaptation was an exclusive function of the rods. Only after the measurements of foveal cone adaptation had been made (Hecht, 1921) was the reason for this apparent: cone adaptation is so fast that Piper missed it completely. The confirmation and extension of these results by Kohlrausch (1922) emphasized the existence of both cone and rod dark adaptation, and showed that the two are to a certain extent sharply separated in time.Following adaptation to ordinarily bright lights, dark adaptation occurs in two parts. The first begins at once; it is rapid, and is due to cone function. The second part shows up somewhat later; it is slow, and is due to rod function. Under these circumstances, cone adaptation is over in 3 or 4 minutes, whereas rod adaptation takes at least 30 minutes. The intensity range covered by the rods and by the cones during dark adaptation depends on the color of the measuring light (Kohlrausch, 1922;1931), on its area and retinal location (Hecht, Haig, and Wald, 1935), and on the duration and intensity of the preceding light adaptation (Miiller, 1931;Wald and Clark, 1936;Winsor and Clark, 1936;.

The relation between visual acuity and illumination was measured in red and blue light, using a broken circle or C and a grating as test objects. The red light data fall on single continuous curves representing pure cone vision. The blue light data fall on two distinct curves with a transition at about 0.03 photons. Values below this intensity represent pure rod vision. Those immediately above represent the cooperative activity of rods and cones, and yield higher visual acuities than either. Pure cone vision in this intensity region is given by central fixation (C test object). All the rest of the values above this transition region represent pure cone vision. In blue light the rod data with the C lie about 1.5 log units lower on the intensity axis (cone scale) than they do in white light, while with the grating they lie about 1.0 log unit lower than in white light. Both the pure rod and cone data with the C test object are precisely described by one form of the stationary state equation. With the grating test object and a non-limiting pupil, the pure rod and cone data are described by another form of the same equation in which the curve is half as steep. The introduction of a small pupil, which limits maximum visual acuity, makes the relation between visual acuity and illumination appear steeper. Determinations of maximum visual acuities under a variety of conditions show that for the grating the pupil has to be larger, the longer the wavelength of the light, in order for the pupil not to be the limiting factor. Similar measurements with the C show that when intensity discrimination at the retina is experimentally made the limiting factor in resolution, visual acuity is improved by conditions designed to increase image contrast. However, intensity discrimination cannot be the limiting factor for the ordinary test object resolution because the conditions designed to improve image contrast do not improve maximum visual acuity, while those which reduce image contrast do not produce proportional reductions of visual acuity.

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Visual purple from winter frogs shows an intermediate yellow color during bleaching by light; summer extractions do not. This seasonal effect can be duplicated by variations in the hydrogen ion concentration and in the temperature of the solutions. Increasing the pH approximates the summer condition, while decreasing the pH approximates the winter condition. Temperature has no effect on the bleaching of alkaline solutions but greatly influences acid solutions. At low temperatures the bleaching of add solutions resembles the winter condition, while at higher temperatures it resembles the summer condition. A photic decomposition product of frog retinal extractions is an acid-base indicator: it is yellow in acid and colorless in alkaline solution. Its color is not dependent upon light. The hydrogen ion concentration of visual purple solutions does not change under illumination, nor is there a difference in the pH of summer and winter extractions. Bile salt extractions of visual purple are usually slightly acid. The conflicting results of past workers regarding the appearance of "visual yellow" may be due to seasonal variation with its differences in temperature, or to the presence of base in the extractions. It is also possible that vitamin A may be a factor in the seasonal variation. The photic decomposition of visual purple in bile salts solution, extracted from summer frogs, follows the kinetics of a first order reaction. Visual purple from winter frogs does not conform to first order kinetics. Photic decomposition of alkaline, winter visual purple extractions also follows a first order equation. Acid, winter extractions appear to conform to a second order equation, but this is probably an artefact due to interference by the intermediate yellow.