Everybody knows what the blind spot is, and every psychology and
physiology text-book talks about it. But as a rule it is identified only
with the little point and the tiny cross pictured in the text-books, and
it is supposed that it does not much matter if the little cross, under
certain circumstances, can not be seen. But it must not be forgotten
that the size of the blind spot increases with the distance so that at a
fairly great distance, possibly half the length of a room, the blind
spot becomes so great that a man’s head may disappear from the field of
vision. According to Helmholtz: “The effect of the blind spot is very
significant. If we make a little cross on a piece of paper and then a
spot the size of a pea two inches to the right, and if we look at the
cross with the left eye closed, the spot disappears. The size of the
blind spot is large enough to cover in the heavens a plate which has
twelve times the diameter of the moon. It may cover a human face at a
distance of 6´, but we do not observe this because we generally fill out
the void. If we see a line in the place in question, we see it unbroken,
because we know it to be so, and therefore supply the missing part.”
A number of experiments have been made with more or less success to
explain the blind spot. It is enough for us to agree that we see
habitually with both eyes and that the “spot as big as a pea” disappears
only when we look at the cross. But when we fix our eyes on anything we
pay attention to that only and to nothing else. And it is indifferent to
us if an uninteresting object disappears, so that the moment we begin to
care about the “spot as large as a pea,” it is immediately to hand and
needs no imaginative completion. If it be objected that fixing with the
eyes and being interested are not identical, we reply that a distinction
is made only in experiment. You fix one point and are interested in the
other because you expect it to disappear. And this experiment, as
anybody will immediately recognize, has its peculiar difficulty, because
it requires much concentration not to look at the point which interests
us. This never happens in the daily life, and it will not be easy to fix
a point which is not interesting.
At the same time there are conceivable cases in which objects seen
askance may be of importance, and where the visual fixation of a single
point will not reveal every reflection that fell on the blind spot. I
have not met with a practical case in which some fact or testimony could
be explained only by the blind spot, but such cases are conceivable.