In this paper the contribution of the accommodation to some visual tasks, particularly in the range of low luminous levels, is considered. Theories are described regarding accommodation as well as the changes produced in the eye during the accommodation process. The visual process, when the eye is adapted to darkness and the luminous stimulus has luminances not bigger than a given value, is described as well as the phenomena of night myopia and presbyopia, which appear when the eye is working under such conditions, and the influence which both phenomena have on the visual performance. These latter effects are related to the mechanism of accommodation, since it involves a loss of the accommodative faculty. A critical study is given of the causes proposed by different researchers in order to explain the phenomenon of night myopia, and finally experimental evidence is presented proving that the main cause of night myopia is closely related to the mechanism of accommodation. Measurement of the size of the Purkinje images by photographing the eye in full darkness provides a measure of night myopia. It was observed that the position of rest of the eye-lens, i.e., the position it takes when there is no stimulus to put the mechanism of accommodation in action, is not focused to infinity but to a distance of about 0.8 meter. The discovery of this rest position of the eye gives a more logical construction to the Helmholtz-Gullstrand theory of accommodation.
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