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Section 39. (3) The Blind Spot.

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 textbooks, 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.