The absolute visual threshold of a field on zero background covering a given retinal area in the fully dark-adapted eye depends on at least three factors: the proportion of the incident light absorbed by the photosensitive pigment, the summation properties of the retinal area concerned, and the number of acting quanta necessary to evoke a visual response. This number may, of course, depend on the distribution, in space and time, of the quanta. The relationship between the areas and the thresholds of fields centred at a given retinal position has often been used as the basis for theories about the number of acting quanta required, but such deductions depend on assumptions-usually of uniformity of the retinal areas concernedabout the first two points. The present experiments were undertaken to test the validity of such assumptions, and to investigate the possibility of formulating a plausible theory of visual performance in threshold and simple acuity experiments on the basis of 'visual units', groups of receptors acting together. The experiments on acuity are described elsewhere (Hallett, 1961).
METHODSApparatus. Figure 1 is
Various geophysical factors have been blamed as the cause of a rare blinding disease found in the Red Sea littora. A small expedition of the Scientific Exploration Society backed by The Royal Military College of Science in England, whose laboratories were utilised, visited the Dahlak group of islands. There it was discovered that in some areas human eyes are subjected to relatively high short ultra-violet radiation (SUVR) doses. This was true not only for total integrated ultra-violet radiation but also in the case of that proportion reflected from white coral sand especially when contaminated with salt. The day-long activities of the inhabitants, whose living depends on diving and fishing, in islands without shade, as well as the weather conditions, contribute to a complexity of circumstantial evidence strongly supporting the view that the condition known as climatic keratopathy arises from the cumulative effect on the exposed part of cornea, especially in the range discovered there -290 to 310 nm. Laboratory work was carried out on samples of topsoil on the islands and in the mainland of Eritrea. Ultra-violet sensors and detectors were used under different circumstances to augment the information. This paper amplifies the clinical paper on Dahlak blindness published in the British Journal of Ophthalmology.
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