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.

No abstract

No abstract

The absolute threshold of the human eye has been measured by a number of workers using a small peripheral-test field and a short exposure. Hecht, Shlaer & Pirenne (1942) used a circular test field subtending an angle of 10' at the eye, placed 200 from a fixation point, and illuminated in 0-001 sec flashes. They found that the average value of the threshold for the dark-adapted periphery corresponds to a flash energy content of about 100 quanta of wavelength 0 51 ju striking the cornea of the eye (54-148 quanta for a 60 %-frequency of seeing, depending on the subject). These values correspond to the minimum amount of light energy necessary for vision. Most of the experiments we wish to report here were made under very different conditions, namely using a large test field, about 450 in diameter, exposed to the dark-adapted eye for periods of several seconds without using a fixation point. On account of the large visual angle subtended by the test field and of eye movements, the image is likely to fall in turn on all parts of the retina, including the most sensitive regions of the periphery, during one exposure. When the field is just visible, the total amount of light energy entering the eye during one 5 sec exposure -is of the order of 200,000 quanta. This is much greater than in the case of a small flash, but on the other hand the flux of radiating energy reaching the retinal receptors is extremely low. Only a small fraction of the 20 million odd rods covered by the image of the field can possibly absorb a light-quantum during one exposure.Threshold measurements were made using spectral bands of wave-lengths close to 0-51 , and isolated by colour filters from the continuous spectrum of an electric tungsten filament lamp. By using other colour filters the scotopic sensitivity curve was measured for two subjects, to check the method. The absolute determinations of radiating energy were based on visual photometric comparisons with electric lamps calibrated for candle-power and colour temperature by the National Physical Laboratory. The threshold was also PH. Cxxm 27