The minimum energy required to produce a visual effect achieves its signiticance by virtue of the quantum nature of light. Like all radiation, light is emitted and absorbed in discrete units or quanta, whose energy content is equal to its frequency v multiplied by Planck's constant h. At the threshold of vision these quanta are used for the photodecomposition of visual purple, and in conformity with Einstein's equivalence law each absorbed quantum transforms one molecule of visual purple (Dartnall, Goodeve, and Lythgoe, 1938). Since even the earliest measurements show that only a small number of quanta is required for a threshold stimulus, it follows that only a small number of primary molecular transformations is enough to supply the initial impetus for a visual act. The precise number of these molecular changes becomes of obvious importance in understanding the visual receptor process, and it is this which has led us to the present investigation.The first measurements of the energy at the visual threshold were made by Langley (1889) with the bolometer he invented for such purposes (Langley, 1881). He found the energy to be 3 X 10 -9 ergs for light of 550 m#. Langley worked before the physiology of vision was understood, so that he used the wrong light and took none of the precautions now known to be necessary; even so, his results are too high only by a factor of 10.In the fifty years since Langley there have been eleven efforts to redetermine the minimum energy for vision. We have carefully studied all these accounts and have done our best to evaluate the measurements. Unfortunately, many of them contain serious errors which invalidate them. Most of them involved no direct energy determinations; instead, the investigators relied on previously measured energydistributions in standard sources and made elaborate computations from them. Only a few can be considered as reliable.

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;.

1. A study of the historical development of the Weber-Fechner law shows that it fails to describe intensity perception; first, because it is based on observations which do not record intensity discrimination accurately, and second, because it omits the essentially discontinuous nature of the recognition of intensity differences. 2. There is presented a series of data, assembled from various sources, which proves that in the visual discrimination of intensity the threshold difference ΔI bears no constant relation to the intensity I. The evidence shows unequivocally that as the intensity rises, the ratio See PDF for Equation first decreases and then increases. 3. The data are then subjected to analysis in terms of a photochemical system already proposed for the visual activity of the rods and cones. It is found that for the retinal elements to discriminate between one intensity and the next perceptible one, the transition from one to the other must involve the decomposition of a constant amount of photosensitive material. 4. The magnitude of this unitary increment in the quantity of photochemical action is greater for the rods than for the cones. Therefore, below a certain critical illumination—the cone threshold—intensity discrimination is controlled by the rods alone, but above this point it is determined by the cones alone. 5. The unitary increments in retinal photochemical action may be interpreted as being recorded by each rod and cone; or as conditioning the variability of the retinal cells so that each increment involves a constant increase in the number of active elements; or as a combination of the two interpretations. 6. Comparison with critical data of such diverse nature as dark adaptation, absolute thresholds, and visual acuity shows that the analysis is consistent with well established facts of vision.

Expected ResultsOur previous studies of the relation between light intensity and critical fusion frequency (Hecht and Verrijp, 1933 b) have shown that the differences which the measurements exhibit when they are made in different retinal locations are an expression of the duplex structure of the retina (Schultze, 1866;Parinaud, 1885;yon Kries, 1929). With a centrally located 2 ° field the data are continuous over the whole intensity scale, and may be described by a simple sigmoid curve, whereas with a peripherally located 2 ° field the data divide sharply into a low intensity section and a high intensity section each of which may be described by a single curve. Since the central, 2 ° field falls within the rod-free area of the retina, the continuous nature of the data indicates that they are a function of the cones alone. The double nature of the peripheral measurements very likely represents rod function for the low intensity section and cone function for the high intensity section. This is borne out by the increasing separation of the two sections as measurements are made farther and farther from the center: the cone section shifts to higher intensities and the rod section to lower intensities, as would be expected from the increasing ratio of rods to cones in these regions.In order to confirm the identification of the two sections with rod * A preliminary report of these measurements was made to the Optical Society

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