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One is liable to get the impression that, in eyes with perfect vision, only focused light reaches the retina, although the incorrectness of such an impression is readily demonstrated in various ways.
If we close the lids in any strong light, we can demonstrate that sufficient light reaches the retina through them to give quite accurate quantitative perception; and about the same amount of this light reaches the retina through the lids and the sclera when the eyes are open and in use for distinct vision. The ability to light up the interior of the eye by oblique illumination through the sclera or iris, we have all demonstrated. But skiascopy has shown that even through the pupil, always in the dilated pupil, often in the pupil as contracted by the ordinary daylight, a considerable part of the light entering, often the greater portion of it, reaches the retina unfocused.
JACKSON E. THE VISUAL ZONE OF THE DIOPTRIC MEDIA AND ITS STUDY BY SKIASCOPY.Read in the Section on Ophthalmology, at the Forty-fifth Annual Meeting of the American Medical Association, held at San Francisco, June 5-8, 1894.. JAMA. 1894;XXIII(9):342-345. doi:10.1001/jama.1894.02421140016002d