CURIOUS OPTICAL ILLUSION.
One of the most curious facts relating to the science of vision is the absolute insensibility of a certain portion of the retina to the impression of light, so that the image of any object falling on that point would be invisible. When we look with the right eye, this point will be about fifteen degrees to the right of the object observed, or to the right of the axis of the eye, or the point of most distinct vision. When looking with the left eye, the point will be as far to the left. The point in question is the basis of the optic nerve, and its insensibility to light was first observed by the French philosopher, Mariotte. This remarkable phenomenon may be experimentally proved in the following manner:
Place on a sheet of writing-paper, at the distance of about three inches apart, two colored wafers; then, on looking at the left-hand wafer with the right eye, at the distance of about a foot, keeping the eye straight above the wafer, and both eyes parallel with the line which forms the wafers, the left eye being closed, the right-hand wafer will become invisible; and a similar effect will take place if we close the right eye, and look with the left.
ANOTHER.
Cut a circular piece of white paper, about two inches in diameter, which affix to a dark wall. At the distance of two feet on each side, but a little lower, make two marks; then place yourself directly opposite the paper, and hold the end of your finger before your face, so that when the right eye is open it shall conceal the mark on your left, and when the left eye is open the mark on your right. If you then look with both eyes at the end of your finger, the paper disc will be invisible.
ANOTHER.
Fix a similar disc of paper, two inches in diameter, at the height of your eye on a dark wall; a little lower than this, at the distance of two feet on the right hand, fix another of about three inches in diameter; now place yourself opposite the first sheet of paper, and, shutting the left eye, keep the right eye still fixed on the first object, and when at the distance of about ten feet, the second piece of paper will be invisible.
Or, fix three pieces of paper against the wall of a room, at equal distances, at the height of the eye. Place yourself directly before them, at a few yards distance, close your right eye, and look at them with your left, when you will see only two of them, suppose the first and second; alter your position a second time, and you will see the second and third, but never the whole three pieces together.
On a sheet of black paper, or other dark ground, place two white wafers, having their centers three inches distant. Vertically above the paper, and to the left, look with the right eye, at twelve inches from it, and so, that when looking down on it, the line joining the two eyes shall be parallel to that joining the center of the wafers. In this situation close the left eye, and look full with the right perpendicularly at the wafer below it, when this wafer only will be seen, the other being completely invisible. But if it be removed ever so little from its place, either to the right or left, above or below, it will become immediately visible, and start, as it were, into existence. "It will cease to be thought singular," says Sir John Herschel, "that this fact of the absolute invisibility of objects in a certain point of the field of view of each eye should be one of which not one person in ten thousand is apprized, when we learn that it is not extremely uncommon to find persons who have for some time been totally blind with one eye without being aware of the fact."