Are Eyes ever Autophanous?

Abstract

I VENTURE to suggest the following simple explanation of the phenomenon described by Colonel Herschel in NATURE of January 18. At the distance at which his experiments were made the light from the lantern or other source enters the lens practically as a pencil of parallel rays, which is concentrated as a bright spot on the retina. The divergent light from this bright spot, which passes backwards through the lens, is again made into an almost parallel pencil. If the retina is exactly in the focal plane of the source of light, and the image an absolutely sharp one, the whole of the light is again concentrated on to the bull's-eye of the lantern, and without the use of a transparent, but partially reflecting, surface, it must be impossible to see it. But the least blurring of the image, from whatever cause, leads to a slightly divergent pencil, which, however, is still narrow enough to concentrate the returning light within a degree or so from the direction of the original source. To an observer within this cone of rays the bright spot on the retina will appear to fill a part or the whole of the pupil, just as a black dot may be made to fill the whole aperture of a lens to an observer at a distance by holding the lens at its focal distance from the point in question.