On a sheet of black paper, or other dark ground, place two white
wafers, having their centres 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 centre 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. The distances here set down may, perhaps, vary slightly in
different eyes.
Upon this curious effect, Sir John Herschel observes: “It will cease
to be thought singular, 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 apprised, 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.”