Popular Mechanics, huhtikuu 1916
What is without doubt a sound explanation of certain nocturnal apparations has been advanced by a German scientist who attributes the responsibility for this phenomenon to the peculiar functions of the eye.
Many persons far removed from superstitious influences have encountered phantom images in the dark. They have awakened in the middle of the night and been startled by the presence of a dim, glowing light at one side of a room. An when they have turned their vision directly toward it in an attempt to determine its source, it has mysteriously vanished, only to reappear weirdly in another spot an instant later. The presence of an unpleasant something is undeniable, yet to tell what it is and to determine its exact location may at the moment be impossible. When such an incident is experienced by one who is only half awake and possessed with a healthy imagination, it is not to be wondered that a sense of right and bewilderment often creeps into his mind.
Millions of very minute fibers which are branches of the optic nerve constitute the retina of the eye. At the end of each of these tiny members is an organ which converts light into nerve excitation. This organ appears in two forms, one known as a cone and the other as a rod, each different in both structure and purpose. it has been found that the so-called cones are active only in strong light, as during the daytime, while the rods are brought into service at night when there is scant illumination. Thus the functions of the two organisms are opposed so that one rests while the other works. At the point on the retina where an image is focused the sharpest, a depression employed when one centers his vision upon an object to get the clearest possible view of it, only cones are provided. Since these function only in strong light this part of the retina is sightless at night, which explains the reason that the shapes of objects seen in darkness are not well defined nor distinct. Outside of this focus spot, however, the cones and rods are both distributed over the retina.
Although the cones convey color impressions to us, the rods do not. The latter are literally color-lind and are able to transmit only white, gray, and black. That is why reds appear black to us at night and certain blues and greens seem to be whitish in color, or "rod white", as it is termed; an indistinct, silverish light similar to that of the stars visible on a dark night. Thus it will be understood that, when in an unilluminated room, we are entirely dependent upon the colorblind rods of the retina for sight, and that the sharp focusing point, or central depression, of the latter is off duty.
And here lies the reason that people frequently imagine that ghosts have hovered over them in the darkness, or danced about them in a lonely graveyard at midnight Suppose, for example, that a pale shaft of light were redlected from some invisible source upon any object in a room, very dimly illuminating a sufficiently small area so that the image of it would barely cover the focus point of the retina. The rays from this, it should be asumed, would not be strong enough to affect the cones, but would excite the rods. Thus if one's vision were directed at one side or another of the light sport, the rays would hit that part of the retina provided with rods. The illuminated image would then be visible. Upon seeing an unnatural glow in a room after being roused from slumber it would be but natural for an observer to attempt to scrutinize it carefully. in other words he would attempt to focus his eyes upon the pale surface. The image in turn would move over the retina. The instandt that it covered the rodless spot it would vanish from view, for the cones would be unaffected by it. Realizing that "the thing" has mysteriously disappeared, the observer might shift his glance slightly, and there would be the ghostly spot glaring at him as before. Again it would melt into nothingness and hide in the folds of darkness at the least provocation.
This explains one of the much hooted-at, feared, and talked-of apparations. it is one that might readily serve as a glowing, beckoning finger, as an "evil eye," or an evasive will-o'-the-wisp. And it is accounted for by the limitations, or peculiarities, of the human eye.